I can't do a full pullup either. A couple of weeks ago I couldn't even really do a chin-up (though I used to be able to). I just did assisted / negatives, which for me means... Jump! Then lower yourself down as slowly as you can. And jump a little bit less every time until you can do it without using your legs at all.

And then once you can do it from standing level, you work up to doing it from a dead hang somehow. I'm hazy on the details there because I've never gotten that far myself.

Assuming you don't have access to a gym, one thing to try is to obtain a strong theraband. Hold the band with both arms out in front of you, elbows slightly bent, and extend your arms out to your sides. You should feel this in the upper back muscles. If this gets easy, double up the band. Eventually that should provide enough upper back strength to try a pull-up. (You'll also need to do some biceps training, which you could also do with an anchored theraband, or with household objects, if you don't want to obtain dumbbells.)

If you have access to gym equipment, then pull-downs with a lat bar and bent over rowing train most of the muscles you will need.

Wide push-ups will get you partway there, which the same tool is actually pretty good for (flip it over on the floor).

If you have easy access to a gym with machines (e.g. if you are a student), one of those machines is hopefully an assisted pull-up machine which will let you add counter-weight as necessary.