We work with people every single day who say "I want to get better at pull-ups." The most effective, and really the only way to improve your ability to get your 1st pull up (or 100th) is to consistently work on them - outside of class time. In class you'll routinely squat, deadlift, run etc. and will get good at those movements. The pull up, however, requires extra attention as for most of us, it is not a natural movement for us or one we're not accustomed to. Below is a PDF progression for you to utilize 3x per week that will take ~ 5 minutes and is guaranteed to help you improve your pull-ups, regardless of where you are on your journey. Here's to consistency and going to work on your weaknesses!
Bonus note: The second best way to get better at pull-ups is to lose weight. I'm not saying this to encourage you to eat cleaner however if you drop 5 lbs with a little Intermittent Fasting or clean eating over the course of a week, you'll notice a HUGE difference in your ability to pull. Ask any of these people who have lost weight and they'll tell you.