I agree with David's response in keeping settings at default for now. For other users who have been using the program for a while, it would also depend on your setup (other security software on your system). If you look at my Signature as an example, I cannot put my Avast settings on high with all shields as this may cause a conflict with other software and as of now all my software play well together. I have tested and tried putting things up higher for extra protection on certain software but it only disables other software, so I have a happy medium right now.
Keep in mind that everyone has a different set up, so what works for one person may not work for another. But using default settings initially for Avast is recommended as previously stated.