What do i do if i have had smegma build up and itching on penis? the itching only on underside of foreskin?

Smegma irritation: Build up of smegma indicates poor penile hygiene. Combination of smegma & sweat or urine is very irritation to delicate inner foreskin. Suggest you retract foreskin & wash penile glans & inner foreskin once or twice a day with bland soap. That will take take care of your problem. Otherwise you can always get a circumcision & penis will self clean when you shower or from rubbing against underpants.