Like any move to a new program there is a steep learning curve from what you are used to (it's just different), browsing the avast help file will help, as will the sticky topics at the top of each of the forums. They provide a wealth of information to help you get the best from avast.
The default settings provide a balance between performance and protection, so you should be OK with that whilst you learn a little more about avast. avast has a number of resident (always on) on-access scanners these scan all newly created and modified files and executables prior to execution, so you should be well covered.
Updates are automatic and incremental so the download size is relatively small and quick, right click the avast icon, Program Settings, Update (Basic).
Right click the avast icon, select Start avast! Antivirus, here you can do on-demand scans, because of the various resident scanners this isn't critical. I do a weekly On-Demand, Standard scan (without archives) of the Local disks (all hard drives).
