Selling my phone - what do I need to do?

I have Avast Mobile security and Avast anti-theft installed on my Samsung Galaxy S3? What steps do I need to follow in order to be able the phone? Sorry if it’s a really dumb question I just haven’t seen it answered anywhere.

I mean sell not able my phone!

Remove it, unroot it (if its rooted) make a factory reset and flash new original firmware.