Welcome to the forum!
There is no such thing as a “factory reset” — not on the x86
platform anyway. If you want to start off with a clean slate, then you’re going to have to wipe everything off of the internal drive. Commonly, this is done by firing up a partitioning tool and erasing the partition table.
After that, you can start installing an operating system of your choice, be it one or several GNU/Linux distributions, Microsoft Windows, or whatever.
Do however bear in mind that Manjaro is not suitable for absolute beginners. Please read the following short essay to understand why.