Android phones support multiple masks running in the same time. It’s hard to keep track of tasks and to-dos on a simple notepad which is not in front of you at all times. And then how to kill anything that's using too many resources and not being used?
1. Android OS has its own task manager. It can adjust automatically your tasks running under the background according to priority level.
2. Download a program made to kill tasks. You can find many this kind of task killer apps from internet.
Here is a link for you to download one: