How I Stop Apps From Running In The Background On My Android

Now whenever I click on the menu button and go to ‘manage apps’ section and then go to the ‘running’ apps tab, I see many of the apps running in the background and using around 270-280 MB out of total 512MB RAM.

I do not want all the apps to be running and using RAM and wi-fi at all the times. I just want to launch these whenever I want and then close down completely when I close them without their automatic relaunching with phone restart. Please guide me.