Just bought my panp5, sysem76. I need a mentor!

I'm an Ubuntu newbie. Switching from xp isn't bad, but some of the simple/basic things are hard to find in the forums or "help" guides. ( or I don't know the right words for the questions!). Example:

I cant find the setting for erasing my internet browsing history, junk files, and other whatever during each boot-up.
What are the mandatory start-up programs, and how do I streamline up that file?

What kind of security risks are associated with Linux, I was under the assumption that it was super safe. If not, what kind of daily maintenance can I do to protect my system?

Also, running some of the code in the terminal can be a bit nervy, as instructions are not always clear 'when to type', 'when to hit enter', or what to do if a mistake is made. I'm aware that there is no "undo" button in Ubuntu!

The browsing history thing is most important, but any help would be great.

Thanks