RTS'... Not hard to get into, but hard to get good at...
You have to learn how to react and respond to whatever comes your way in such a manner that it effects you only minimaly. Also, it's important that you muster the skills of using hotkeys... Thus creating a big time-advantage between you and an 'ordinary' player who hasn't taken the time to learn those... Also, learn the tech-tree... Learn in which sequence you have to build to be the fastest on the top of the food-chain. And don't forget! ALWAYS expand your base, ALWAYS search for more resources... If you just build an army and go in defensive mode you may hold for a short amount of time, but you have lost anyway then... It's only a matter of time! Always expand...
EDIT: ecurbj, if you wish to play the game online you'll always have better players then you because they are already familiar with the RTS genre. However, if you buy the game (in this case Halo Wars) at launch then you won't have that big of a disadvantage... Oh, and the game is coming out on X360, only a few gamers have experience with console-RTS's (from LotR:BFME).