Now I've finally edited this old thing out.
Anyway, there is this awesome half life mod called Natural Selection. It's a hybrid of a FPS and a RTS. It's free right now and downloadable at http://www.unknownworlds.com/ns/.
I can't explain much right now, but go to the manual at http://www.unknownworlds.com/ns/comm_manual/index.htm to understand how things work.
There are two types of mode and they are Classic mode, where you must gather resources and support each other to defeat the other team. In combat, there are no structures(may change due to server mods), no need to gather resources, but levels for each player.
The two teams are marines, which are easier for new players usually. And aliens, which are a little harder to figure out at first.
If you want to play the game, I'd advise you to try out marines first, in either mode is fine. If you're a marine, just follow the commander's orders, or whatever everyone is doing. If you're an alien, you should try playing combat mode first, and follow everyone's orders.
So check it out!