Okay. In that case, a few tips.
First and foremost, until you get s4 gear with epic gems you have to play VERY offensive. I'd go so far as to say that you should play disc (unless you absolutely don't want to or you have your heart set on shadow). Shadow priests around that time had three problems; mana, survivability and utility. Namely, you went OOM very quickly if you popped out to heal, you couldn't keep yourself up without someone to peel (hence why you HAVE to play with a rogue (mages don't do enough damage), and you don't bring much to the table outside of MC and silence.
So with that being said, you have to play very offensively. Essentially, you're rotating CD's around your Silence and trying to get kills with Silence on the healer. You can dispel through everything and given downranking you should be spamming dispels. Keep in mind you can do damage with DoT's through a MC so use it to great effect. The main thing that distinguished good spriest teams from bad ones were the use of MC and how good your teammate is. Be prepared to be frustrated; shadow at that time was exclusively reliant on your partner. You could be a world class spriest and still sit in Rival range if your partner wasn't good.
You should pretty much never be healing unless absolutely necessary, shields should nullify most of the damage coming out and you want games to be quick lest you die (you can't outlast). Once you get up to s4 gear I'd suggest mixing in some PvE gear for maximum burst. Remember your rogue should easily lock the DPS target on you down while you get away and from there you need to get a kill ASAP.
You should mostly be using instants until you get ample time to get a cast off. Unlike post-Wrath, VT is not your main objective. People can and will lock you out and if you're sitting there trying to get VT off you're not being effective. SWP + MF can do a lot of damage and you can wait to chain a MB/SWD into a bursty kill. You should be mind flaying melee (it slows them down and you have DoT coverage and druids/shamans can't dispel!). Always dispel everything on yourself, your teammates and your opponents (I'd go so far as to have 4 dispel macros for each of those targets). Downranking is your friend, heals are not.
Don't go for survivability in either spec, gems or playstyle as you will lose every time if you do. Finally, you need 450 resil at top of the line gear (s3). 350 resil should be a minimum if everyone else has s4. Luckily it's not a unsurmountable jump when going up seasons (as opposed to now/Wrath where new seasons gave stat increases that largely nullified the previous seasons gear).