Getting a wonky rear derailleur dialed in is not easy. Lots of things can mess you up before you even start making adjustments - a bent hanger, a weak spring, worn pulleys, a bad shifter, bad cables/housings, a bent axle, frozen limit screws/barrel adjuster, or improperly adjusted cables.

If all of that is good, the actual adjusting part can be accomplished with a stand, a YouTube video, and a couple of simple hand tools.

Also worth noting that no derailleur shifting at all is still a rideable bike, and bad shifting is generally still better than no shifting. I had a derailleur once that I had to kick with my right heel like I was spurring a horse every time I wanted to get out of "first" gear. It still got me to work until I got around to replacing it.