Malicious software runs on the background of Android phone, but I cannot see it - how to find and remove it?

I know that a malicious malware/spyware has been installed on my Samsung Galaxy S3 phone and I constantly get the message to close apps not in use because they drain my battery. But I do not have anything running. I have about 3-4 apps, others were turned off/updates uninstalled a long time ago. So the questions are not if my phone is infected - I know it is. The question is, how to find and remove it?