I struggle with pull ups :( any help to increase them?

I can run really fast I'm not overweight and I can do a lot of push ups sit ups , and I play soccer pretty well I even learned how to play forward , but my friend who is fatter and has less muscle than me can do way more pull ups , we went out for a jog he smokes too so I was able to run way faster but when it came too doing pull ups I could barley do one , I also increased my bicep and back muscle to help me and still nothing I can do bended elbow pull ups but not all the way down ones :(