Step 1:

Depending on the version of Android your device is running, tap on the menu button, and select “Settings.” If you’re device doesn’t have a menu button, move on to Step 2.

Step 2:

Launch the Settings app on your device, or tap in "Settings" if your device does have a menu button.

Step 3:

Tap “Applications” or “Apps.”

Step 4:

Tap “Manage Applications” or go to Step 5 if the option isn’t present.

Step 5:

Select "Running" at the top of your Manage Applications window.

Step 6:

View your list of running applications.

Step 7:

Select the first app that you are not using and want to close.

Step 8:

Tap "Force Stop" or “Stop” and hit your "Go Back" button to view the list again and select your next app.

Step 9:

Continue down your list until you've closed all the apps you want to.