Why= pushup is tricep petoral muscle groups, pullups are bicep lats and mid back muscles. Now patiently do curls and bent over rows for around a month building reps and wieght then you've prepped for pullups. Also like anything maybe get a stool and stand under bar and slowly lesson legs assist and also negative reps will build muscles so in chinup position lift up legs and slow and controlled lower yourself, then stand and repeat. You can pullup using just some leg help so you can just barely do it. In 3 months you should be able to do at LEAST 5 hopefully more if you're diligent in your training of every day with assistance but every other day without so your muscles heal between work outs. Never overdue it as in working past pain but maybe once a month then give yourself 2 or 3 days off. Now almost to pain but not unrepping it you can workout daily, but also work out the opposite muscles as in benchpresses or pushups equally so your not to strong on one part as the opposite part, balance is key to all.