Ah-Ha! A new underlin..."Friend" to show-show the ways of the Man-Rats! Wewewewelcome!
I can't speak Skavenese very well
So, Skaven are a very defensive army, unless you want to run Moulder, in which case they become super aggressive! Here are a few tricks for you:
(Ive split these up into smaller sections, i don't know about all the units, so ill just talk about what i know)
Masterclan:
Grey Seers are useful, but i tend to lead towards Warlocks to fill my wizarding needs. They do have a nice command ability though.
Verminlords. Whats not to love? Taking one can be fun, but a better option is the
SCREAMING BELL! These things are awesome! You've got yourself a Wizard, Leader with a great command ability and a bucket load of attacks for only

260 points. And it can summon Verminords, make your Underlings fight harder or blow itself up! Whats not to love?
Verminus:
Warlords are a basic melee leader. Gnash-Gnaw on their bones! is ok, and I run one in bigger games, but a Masterclan Hero is a better general.
Chieftain with Battle Standard. A nice defensive character that can hold its own in Close Combat. Useful for keeping your Clanrats in line.
Clanrats are your rank-and-file dudes. Take the Rusty Blades, +1" of range isn't worth losing 4+/4+ and making it 5+/4+. Their bravery (4) is their main weakness, so keeping a
BSB nearby is useful. Use them to defend your Warmachines and/or characters.
Skavenslaves Don't take them

. Thy were useful in
WHFB, but suck in
AoS.
Skyre, aka The Fun Stuffs
Warlocks and Archwarlocks are decent spell casters, i prefer Warlocks because of their accumulators, that give you a 18",
d6 mortal wound spell (If the Casting attempt fails the caster suffers d3 mortal wounds instead, yay!). I run at least one, and have run up to 4 at one time (in
really big games). They make good bodyguards for Seers, Warlords and anything else you want hanging back.
Weapon Teams All of these guys have a special-special "POWER UP" (Aka "Super Saiyn) mode, that makes them better, or dead if you fail the dice roll. Fun.
Warpfire Throwers are good mid-ranged weapons. At 60p they are great at filling in that last points gap to make your list an even number.
Warpgrinders are interesting. They can be helpful for getting hard hitting units like Plague Monks or Gutter Runners up close and have a decent melee attack.
Ratling Gun. On paper, this guy is amazing. In games, not so much. Again, nice for points filler but I prefer Warpfire Throwers with their
d6 mortal wounds.
Doomflayer Never used one. Sorry.
Poisoned Wind Mortar A good, cheap artillery piece for taking out single targets (i.e. Heros or Warmachines) at range. One of them can be useful, but i wouldn't bother taking more than 2.
Jezzails Are too expensive (points and money) for what they do. I sometimes run them to take out edit units, but normally they just miss and then die.
Stormfiends Again, haven't used zem.
Warplightning Canon TAKE
IT! The chance of rolling a 1 for power and getting 6 1+ mortal wounds is to much to withstand. I (almost) never leave home without one!
Skyre Acolytes Don't know if these are useful. On paper, they seem weak, but they might be fun in bigger games, for Warmachine/Wizard hunting, maybe with a Warpgrinder to back them up? Or Stormfiends? Dunno...
Moulder
Packmaster Keeps your beasties in line! Thats about all their good for.
Rat Swarms Meh. Don't take them unless you really want to.
Rat Ogres, or Rogers Good suicide units, they can take down big units of heavily armed infantry (read: Ironbreakers, Greatswords, Black Guard, Swordmasters, Blood Warriors, Grave Guard...) and with a Packmaster they can be hard to get rid of.
Giant Rats.[b] These things seem weak. But they have a rule called Wave of Rats, so a unit of 30 rats gets 30 2+/5+ ATTACKS! WITH A PACKMASTER, THIS BECOMES 30 1+/5+ ATTACKS! AND WITH 50, YOU GET [b]150 1+/5+ ATTACKS So take them if you want, but only in big groups.[/b]
Hellpit Abomination I have no idea what this thing does. Ive never used one.
Pestilence:
Remember to keep these away from your non-Nurgle units. Trust me
Plague Priests are mediocre ("Good" in Skavenese) priest characters. Take them to back up your...
Plague Monks! I had a unit of 5 of these wipe out a Phoenix in melee. Give them twin blades, a Book of Woes, an Icon of Pestilence and Bale Chimes and they will deal with almost anything. Their real strength is in charging, getting that extra attack is great for a Skaven unit. Make sure they can finish of the unit they've charged in one turn, or else they become rather... boring. The Icon of Pestilence means that even DEAD monks become a welcome sight, causing a mortal wound on the enemy on a 6+. Good for taking out small, heavily armed units with good weapons.
Plague Censer Bearers. Newbie Plague Monks with better weapons (go figure

). Seem fun, i haven't finished my converted unit so i haven't had time to test them. Sorry.
Plague Furnace. Seems good if you want to bring bucketloads of Plague Monks and other Pestilence units, otherwise the Screaming Bell is better. I magnetized mine, but haven't had a chance to try out the Furnace variant.
Plagueclaw Catapults seem a lot worse than WARPLIGHTNENG CANONS, but remember that target units with more than 10 models increase the damage to
2d6! These are still regular wounds, although with -2, and are 3+/3+, so one miss means its all over. Can be fun against Greenskins and Undead, but a Warplightneng Canon is better for taking down non-horde armies, like your friends Daemons.
Eshin
I have not run anything except
Gutter Runners, that did amazingly, so take them!
Hope this helps!
Ember