It took me 2 weeks to do my first pull up.
I did it by doing... Pull ups.

Initially I didn't have the grip strength to hold on to the bar. So I started with a dead hang. Just trying to hold on to the bar as long as I could.

Next I did negatives. Jump up or something, find a way to get yourself up and then lower yourself down slowly.

Next, doing the actual movement itself.

Also, learn the proper technique. Sometimes it all comes down to technique. Like locking your shoulders in, and inflating your chest.

I got my girl to go through the same thing and she did a pull up and the thing that helped her was locking the shoulder.