Pull-Up Advice

I’m aiming to go to OCS this summer, and I need to work on my PT stats before then. Currently I’m at a 25:00 min 3 mile run, 25 pushups, and a 2:10 plank.

Obviously I need to work on my upper body muscles, my main goal is to go from being able to do 0 pull-ups to at least 6 before April. Does anyone have any tips on how to quickly increase my upper body strength?