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.