Wow, that bike must be the most absolute bottom-end mountain bike ever made. From spot welded dropouts, steel side-pull calipers and rims, to the galvanized spokes. That said, it’s a perfectly rideable bike with a little work. I like old Huffys, I had a blast on them when I was a kid. Your Huffy is in far better shape than the Brompton I just started got in today.
Your bike looks to need nothing but a basic cleaning, cables, and adjustment. You can get the brake and derailleur cables from Amazon for less than $30, maybe $10 for new brake pads. The only tools you need are a wrench for clamping the ends of the cables and installing the brake shoes, and a good cable cutter which can cut your new brake cables without damaging them.
The chain, derailleur pivots and shifter will need to be oiled.
There are numerous videos on YouTube showing how to cut and install new cables, how to change the brake pads, and how to adjust everything. If I wanted a guinea pig bike to use to learn basic bike mechanics, I couldn’t ask for a better one than your Huffy.