Available RAM/applications opening by themselves?

[LittleGreenRobot]

I am a new android user. I have the atrix, obviously. There's said to be a gig of ram, but when I open up advanced task manager, it says I have only 173 mb available, and tons of apps are open, most of which I did not open. Can someone explain this to me? And how to stop these apps for launching automatically. Thanks in advanced.