Focus on really learning how to keep the bike clean, properly lubricated, and how to do a proper safety check before you think about tackling any other maintenance issues. Keeping the bike clean, specifically the drive train, will save you a ton a $$ and other maintenance in the long run.
Not that any of it is that difficult, but you’ll learn quickly that without bike specific tools, bike specific greases/oils, and a bike stand/work space it is simply not worth your time and effort to fool with more complicated fixes.
Investing in some proper cleaning kit and a bike work stand is probably the best place to start.