RAM doesn't need to be cleaned manually. The OS manages RAM automatically.
Or do you want to clean storage instead? If so, try clearing app cache first. I can't offer specific advice on how to do so unless I know which phone you're using.
As @djrakowski indicated, you don't really need to clean RAM. In fact - it's actually detrimental to OS performance to do so. Android wants as much RAM used as possible for best performance, and clearing RAM will actually REDUCE battery life.