What virus protection should I use?
Windows 7 I recommend Microsoft Security Essentials and for Windows 8.1 and 10 I recommend Windows Defender which is built right in to the operating system. Its free and does just as good a job if not better then any of the other free virus protections out there. if you still feel the need for something more I suggest Malwarebytes
How should I back up my personal data so I won't loose anything if my computer crashes?
The best and most assured way to make sure you don't loose your memories and important documents is to store them in the cloud. There are a few choices: One Drive, Drop Box, Google Drive, or Amazon Drive
Do you work on cell phones? Unfortunately I do not, sorry.
Do you buy any used computers or computer parts? Sorry no