If you’re struggling to do your first pull up then a good way to progress is by using negative reps (just the downward part).
To do this you will need to stand on something that allows you to do the negative (downward) part of the pull up, without having to pull yourself up first. Alternatively, you can get someone to give you a “leg up” (as we used to call it back in the day!).
Once you’re in position at the top of the movement, take the strain and slowly lower yourself into a dead hang. Then let go and repeat 5-10 times. Then rest for a couple of minutes and do 2 more sets of 5-10 negative reps.