Orks are primarily a close combat race. They rely mostly on sheer force, so subtlety isn't going to work very well. But they are a very fun race to fight and face, and they respond well even in the hands of cack-handed losers like me!

Their main tactic is "Waaagh!!!" Their seocndary tactic and tertiary tactic is also "Waaagh!!!". But stameding into a fire zone is a bad idea, so look to you deployment and where on the battlefield you set up.
Tactics and Set-up:
WAAAGH!!!
Put big units of orks in battlewagons or a trukk or twelve with some bikers, stormboyz and/or dethkoptas on one flank, big units of footslogging Shootas and a few dreadmobs on the other then advance. The opponent will have to either kill the close-combat death on one flank or the slower nasty stuff on the other. Big Meks, Shootas, Lootas and Flash Gits are good for this tactic in the footslogging role, whilst Slugga Boyz, Nob Squads and Meganobs are insane CC stuff for your battlewagons and Trukks
Da Kunnin Plan
Fast/Flank/Infiltrate/Deep Strike combo with Stormboys, Dethkoptas, Wierdboys, Bikers and a big Shoota/Slugga unit. Upgrade the Weirdboy to a Warphead and try to get the Blink psyker ability. Once you do, DS the big Boyz unit into your opponents deployment zone. Your fast stuff, flankers and/or DSing units will by then show up and complete the mayhem. Very risky, but rather Orky.
Ork Nobs
The Big Biker Boyz (BBB) tactic is simple, Warboss and Nob Unit on Bikes, as big a unit (or two) as you can afford. Charge insanely into the opponent (Turbo Boost to get the cover save when you're far away) and you tend to eviscerate all you encounter. Your standard Bike Mobz (5+ ones are Troops IIRC) can assassinate lone characters, contest or claim areas and do quite a lot of damage of their own.
Uvva Gubbinz
Big Meks, either with a Kustom Force Field or Shokk Attak Gun are excellent "Keep da boyz alive" units, and they will draw fire from what will really do the damage (big units of Boyz), Grots are very good "Ignore me as I claim objectives" units as they are Troops. Docs in your squads will vastly improve thir survivability. In all, there are no real weaknesses to the Orks units, no real "Woo I'm crap!!!" choices like in some codexes. Just try to ensure you get the charge! If you don't your S, I and A all drop, and you get hit a lot harder.