Alex was in her room on Friday afternoon with a few of her friends.
The first sentence is the most important sentence of a whole story. It must draw the reader in. This sentence is incredibly boring. It talks about people we don't know (most of whom are unnamed), in a place you haven't described on a day you haven't described, doing something that the audience doesn't care about.
They were spending the afternoon together during the second semester of freshman year.
If the 1st sentence is the most important, the 2nd sentence is the 2nd most important. Any reader who was willing to look over the 1st sentence and give you a second chance was immediately turned off here. First off, we already know they're spending the afternoon together. Secondly, why should we care that this is the second semester of freshman year?
While Alex was having fun with her friends, she decided to call Kevin, who had been depressed the previous year because he had not had a girlfriend and was not socially engaged.
When someone is having fun with friends, they don't typically think to call depressed people and ask about how they are. Still I'll give you the benefit of the doubt. Secondly, SHOW DONT TELL. Show me that Kevin is unpopular, don't just tell me in an awkward, detached way.
Alex had told him that everything would change when he got to college and that he would have many more friends and that he would find a girlfriend who cared about him. Alex had similar worries on occasion, that if she had never met Heinrich in middle school, that she would never find anyone. Kevin also had difficulty fitting in, and Alex had wanted to help him. She called him while watching her friends play a game. He lived downstairs from her, on the second floor. He answered, and said that he would immediately start coming up. Alex went back to her friends, and eventually started playing Call of Duty World at War with a few of her friends after she mentioned that Kevin was on his way up.
SHOW DONT TELL! Christ, I'm having an aneurysm reading this. The way you're described this scene, it takes place in a vacuum, in which faceless and nameless people play Call of Duty World at War without any emotion whatsoever.
Alex expected the day to be fun, and had good expectations for Kevin that night. The previous weekend, she had hooked up her friend Sasha with an attractive blond girl. Excited by her success and wanting to confirm the encounter, she somewhat creepily followed them back to Sasha’s room where she collapsed outside the door. Her only intent though was to simply confirm that Sasha had hooked up with the girl. This weekend, Alex was hoping to introduce Kevin to girls, and hopefully find him a girlfriend, rather than a random hookup.
This is incredibly awkward. The prose is stilted, the descriptions bizarre (she collapsed outside the door to simply confirm Sasha had hooked up with the girl?) and the characters don't seem even remotely real. Alex exists only for Kevin; she wants him to be happy, she wants to spend time with him, she wants to hook him up with someone, and she's creepy about it too.

Alex was somewhat strange herself, but she still was still fairly normal in many ways. She was not unattractive, but often dressed in ways that made her appear unattractive; she was naturally beautiful, but often was not. She was from East Germany and was half Russian. She had dark red hair, but was only 5’3″, which was not that short, but made her feel inadequate at times. She had started dressing better though, which helped, but she still favored regularity and simplicity. Today she wore a black KGB ushanka, a dark blue sweatshirt, black tight pants, and boots.

When Kevin came up, Alex handed him a controller and asked if he wanted to play. He took the controller and looked for a place to sit down with them. Alex’s room was small and rectangular, with the area where people could gather at a small area near the door. She had deliberately set up the room so that the personal space was at the back near the window, with the group and social area toward the front. She had set up both lofts against the walls toward the end of the room with the desks underneath, and a cart with the refrigerator and microwave at the end of Alex’s loft were it bordered with the open area as possible, while compacting and restricting the private space. The room was not well designed for it, and the open area was used a living area, with a small couch and TV. Everyone who was playing the game was on the couch, and there was not any other open seating; most people just stood. Alex had been diligent to provide seating, and showed Kevin a chair at the end of her roommate’s loft. Kevin took the chair and unfolded it next to Alex, as she sat at the end of the couch. Once Kevin got comfortable, Alex blurted out “What are you doing there? Why are you sitting next to me? Go sit with someone else and start making new friends! You want to break out, don’t spend time with the same people all the time!”
Still, you haven't described anything. Secondly, Alex is a fething psycho. She spends the first whole paragraph obsessing over Kevin, and then she invites him to her room. Then he comes up and sits next to her, and she goes flying rodent gak crazy and humiliates him in front of all her friends.

“Did you just offend Alex by being socially incompetent?” One of Alex’s male friends asked.
I'll understand is English is your second language, but people don't talk like that. If you do live in a mostly English speaking country, I suggest you record the conversations you have during the day, then play them back and write them out. It sounds (and sometimes is) creepy, but it gives you a good idea for what real conversations are like.

“What?” Kevin asked.

“Alex tends to go apeshit on people who don’t engage socially for some reason. It’s not like she isn’t totally awkward herself.”

“I can’t stand it when I see people socially sabotaging themselves! It’s not like it’s that hard! All they have to do is get out there!”

“You don’t seem to have any problem getting out there…” Alex’s friend commented, referring to the reactions she would have to observing behavior she considered unacceptable.
No one talks like this. Secondly, why do you explain what Alex's friend is saying? If you have to explain dialogue, its usually a sign that something is wrong with your conversation.

“Yeah, and I have friends because of it.”

“I don’t want to look like an ass!” Kevin commented. “I’m trying to find a girlfriend, and I don’t want them thinking I’m some fool.”

“Girls would rather have someone who’s funny and who’s confident enough to just go for it. No girl wants to lay the guy who’s so afraid that he stands against the wall all night.”
Alex said.

“You’ve never had to go out with anyone since middle school.” Kevin responded.

“I trust Alex with advice. She has had sex with a woman.”

“What?” Kevin asked.

“I had sex with two girls.”

“Does Heinrich know?”

“Dude…” her friend commented “you need to stop hiding and get out more. Everyone knows about this.”
This conversation is just... really awkward. It feels like when you're watching a movie from the 90's, and people are talking like this: "That's totally radical!" Again, if you live in a non-English speaking country, this is understandable. In that case, I would suggest just watching a lot of English TV and movies. Even if some of the dialogue is cheesy, it'll still help.

Kevin went back to playing the game, were he was promptly shot several times by Alex as she came around a corner. She then started talking about a boy named Allen who was British, had a subtly large head
A subtly large head? I think that by definition, a head can't really be subtle about being large.
, and had a perpetually disheveled appearance with long hair that was directionless and an often unshaved face. He had a very intense military interest, and this was coming from Alex and Heinrich, who spent large amounts of time on stuff related to World War II. Alex thought that it was weird how much he seemed to memorize about modern weapons and vehicles, and how he could quickly recall and comment on them. He also had an odd walk, with a long and scissor like gait and odd hunch with his shoulders, and an inexplicable smile. Alex called it the Jungfrauschrit, after observing it in Heinrich’s older brother. Alex commented on how familiar Allen had been with various militaristic facts.

“You think that’s weird? You’re interested in World War II stuff.” her friend asked.

“I’m not so mono-focused on it. I’ve got other stuff I do, and he sounds like a fact book when he talks about stuff. He sounds like he could be repeating an armored vehicle catalog word for word.”

“I guess that’s somewhat weird.”
This dialogue is so awkward that it makes me cringe to read it.

“So Kevin, are you ready to get laid tonight?” Alex asked, changing the subject.

“Yeah.”

“Yeah? Yeah! What kind of answer is that! You need to show dedication!”

“I am dedicated.”

“You don’t seem dedicated.”
I had to stop reading at this point. I think I might have had a stroke if I kept going