well, its a good thing youre planning on playing large games, because thousand sons are very expensive!
its best to know from the beginning: thousand sons are a very difficult and rewarding army to play with.
best thing to do is to look at your strengths and weaknesses as a TS player:
Pros
- Tough: two wounds each means each squad is effectively twice as hard to kill with basic weapons. look out for S8+ weapons - youll feel their effect very well!
- Rapid Fire: slow and purposeful is great for shooting with rapid fire weapons at 24" while moving. but the real strength is when you get to 12" - let rip with those rapid fire shots

- Fearless: thousand sons are fearless and this is generally a good thing (it may be bad in combat at times if youre loosing very badly)
- Sorcerers: you have access to many sorcerers (all your rubric aspiring champions are sorcerers) and you can have a kick-ass (if very expensive) chosen unit filled with them. choose your powers (and thralls and talismans
) wisely and theyll do well.
Cons
- Expensive: first things' firstm these models so not come cheaply - 24 points for a marine with 2 wounds and fearless is expensive and if used carelessly, not worth it.
- Slow and Purposeful: this is good for shooting with, but can be bad for movement and combat. just make sure you never forget this, and not engage in needless assaults. Also, rubric terminators with power fists are that much better because they strike at I1 anyway.
- Small: the thousand son army is one of the smallest ones you can make. make good use of this to refuse your opponents flank. this may be easier said than done, but the army is quite mobile (insofar as rubric guys can move and shoot at 24", even if they move slowly
also remember vehicles and dreads in paricualr, which have access to many things you otherwise wouldnt have.
remember daemons - they are relatively cheap (the cheapest thing you can field in fact) again, play their strengths (relatively tough, shooty) but be willing to sacrifice them if need be (since they are smmoned they tend to get into a favourable assauty position before the rest of your guys - you can assault them into combat to shield your models form enemy fire.
3-4 chosen with the tzeench shooty power (the name escapes me) three thralls each and a talisman of tzeench (used to reroll failed to hits or wound rolls) are a very good unit. quite pricey but three four sorcerers can shoot 8 S8 Ap(2ish i think) shots a turn, with either a reroll to hit or wound - which is plain nasty!