Like GuitarStv said, it could be your cones. If they become too tight, then the bearings will grind on the hubs and eventually pit the hub. But it's worth a try, new bearing balls will cost you maybe 3$ tops and a bit of grease.
There are quite a few community run bicycle workshop where volunteers will love to show you how to do the work and teach you a thing or two about repairing bicycles. They have all the tools you could need and are usually free or very low priced. Most of the ones where I live are run by universities, cost 20$ a year membership, and have been invaluable in learning how to maintain bicycles.
That said, CT bikes are usually very poor quality. If you have time on your hands, it could be pretty interesting to learn how to fix it but don't expect too much.
Story time: last summer, I was unemployed. I bought a used Specialized bike for 60$ in poor shape. I spent the whole summer fixing it at one of these co-ops, cleaning it, and replacing some parts. I put maybe another 100$ in parts in it. Now I have a bike in pretty good condition, I know how to fix most things on it, and I know what to look for when buying my next bike. It would probably sell for 250$ used now. I did the same thing for my partner: I got her a 20$ bike and overhauled it. It turns out it was too small. We sold it for 120$ a few months after putting maybe 30$ in parts and got her a better one that is her size.
This advice helped me get my first used bike:
https://biodieselhauling.blogspot.ca/2012/01/buying-bikes-from-craigslist.html