Really the Tyranid Tactics thread at the top of this board is a good place to start. I may not agree with all the assesments myself, and some o fthem may have changed a bit since 6th ed has come out and everyone's sought to adapt, but fundamentally it's solid.
The biggest strength to Tyranids is the radical adaptability. It is entirely possible to outnumber your opponents one game, with only a few models that can challenge them for quality, and be outnumbered three to one the following game because you decided to load up on Warriors. You can mix to anything in between, use Mysetic spores to emulate a drop-pod marine force, or just crash across the field with monsterous creatures.
When this codex first came out, I spent close to 6 months loosing as I built army lists around each individual aspect of what's in the codex. From those games I learned (vividly!) the strengths and weaknesses of each part of my force, before trouncing everyone I could find all summer long. I'd suggest you take a similar approach, and work with all the different elements to find uot just what you've really got there.
Lastly, The Tyranid codex is built around Synergy. It's extrodinarily unlikely you can choose just a few model types, say "these are the only units which are any good", and get anywhere with them. Everything has it's role on the battlefield. Termagaunts get in the opponents way, Tervigons draw a lot of well deserved fire, Genestealers make a low profile, Warriors win where you need to win, Tyrants are over-priced support units that your opponent doesn't DARE ignore, Biovores destroy armies, Dues ex Trygon, ect.
Welcome, and happy hunting.
*ps: I almost forgot. Tyranids are a highly psychological army. You gain a lot of advantages from your opponent being afraid, and you have a lot of things they should be afraid of. Keep them blasting away at those Genestealers in a blind panic, and you can snatch an objective (and victory!) with those three surviving Termagants while they aren't looking. Also, paint. Grey plastic isn't nearly as terrifying.