Welcome to the hobby.
Space Marine armies are incredibly flexible with the amount of kits you can get and the types of armies you can field. The best value kit you can get at the moment is the 2nd tier Start collecting box. It will give you another 5 Infernus Marines, brining your current squad up to a full 10. 5 Terminators, as well as a pair of characters to lead them (Captain and Librarian). You'll also get some more Tyranids which can be good for practice games while you're learning the rules.
When collecting beyond that, there are a 3 basic ways to collect pretty much any army in
40k. Value, Generalist, and Specialist.
Value: If your primary focus is spending as little on the hobby as possible while still getting fun out of it, you're going to want to focus on any of the discount boxes that are out. The easiest is the aforementioned Tier 2 starter set, but fortunately, Space Marines have a lot more to choose from. Pretty much each chapter gets a Combat Patrol that can be used in any generic Space Marine force, it just comes with additional upgrade bits for the Chapter in question. Here are some options:
- Blood Angels: This is a good one to start with since you get a solid spread of various other infantry. A standard Intercessor squad that can focus on objectives, an phobos recon squad (infiltrators or incursors) with a Librarian that can join them, a more heavily armoured Aggressor squad, and an Impulsor Transport to ferry them around in. (Personally I think this is the best one for a newer player)
- Space Wolves: Unless you are getting into Space Wolves specifically, I wouldn't recommend this one as the Lieutenant is a specific Space Wolf sculpt and the Reivers this kit comes with have been notoriously underwhelming for the last several editions.
- Dark Angels: While you do get some good units out of this kit: 10 Battleline Intercessors to handle your objectives, 5 Hellblasters to take on Elite Infantry, 3 Bladeguard for close combat, and 1 Gravis Captian. The Captain in the kit, can't actually lead any of the units inside it, so it does require an additional purchase to be usable. (This is a decent second pick after the Blood Angels one though, as the Gravis Captain can lead the Agressors from that kit.)
- Black Templar: This is very much a subfaction specific set, as it comes with their specialist units. Don't get this one unless you want to go with that subfaction.
Generalist: This army is often the rule of cool armies. Just get whatever kits you think look cool and throw them together in a list. Over time, you'll discover what units you like and don't and can tailor your future purchases to whatever your style is.
Specialist: There is a lot of internal synergy between certain types of units in any
40k army and if you know what kind of subfaction or playstyle interests you, you can focus primarily on those kits and build your army around doing that specific thing. Most factions that have received a codex have between 4-7 of these, but Space Marines being the poster child have lost more. Here's what your specialist options are:
- Gladius Strike Force: This is the all rounder option that goes well with a generalist army. It's a good one to practice with if you don't know what you want to focus on.
- Anvil Siege Force: This army likes the big guys with big guns. This force focuses on holding their ground against their enemies and gets buffs based on remaining stationary. So if you like the style of the slow moving but highly durable Gravis armoued infantry, this style focuses on them and anything else that uses weapons with the Heavy keyword.
- Ironstorm Spearhead: If you like Vehicles and Walkers, this is the playstyle for you. Grab a couple Techmarines for support, and whatever vehicles that strike your fancy and you'll have a good time.
- Firestorm Assault Force: This subfaction focuses on getting in close range for shooting and using transports to do it. If you like the flamethrower and melta style weapons and keeping your units safe in transports and popping out at key moments, this is a great subfaction for that. The Blood Angels box plays well with this as you get a transport and the Agressors can be built with flamers.
- Stormlance Task Force: This one is all about bikes and mobility. If you like the look of the Outrider bikes and the ATV's, this is a great pick. Grab some of those with a Bike Chaplain as well as some Jump Pack Intercessors with a Jump Captain and you'll be racing circles around your opponents in no time.
- Vanguard Spearhead: If you like the look of the Phobos units this is the subfaction for them. It's a much more sneaky and tactical take on the army that requires a fair bit of planning and forethought to work. You want lots of the Phobos style recon units backed by Scouts and Invictor Warsuits. This is a tougher style of army to play, so might not be the easiest learning curve for a new player, but it's a lot of fun when you get it down.
- First Company Task Force: This is the best of the best army, the elitest of the elites. You'll focus on the Terminator style units backed by Sterngauard and Vanguard Veteran squads. The Tier 2 start collecting is a great way to get into this as you get 2 terminator characters and a squad, though you will need more to support them.
- Sons of Sanguinius: While most of the specialist factions tie heavily into their own units and builds, Sons of Sanguinius is a good generalist one as well. All your melee units get a big buff. If you like close combat to the exception of all else, this is a great pick. Go for anything that moves fast and hits stuff, with a preference for Jump Infantry and you'll do well here.
Hope that helps you with considering what direction you want to take your initial army in. Keep us posted with what you end up deciding, it's always a good time seeing a new players first army take shape.