Let p be the probability of success and Y the number of separate times one must try to obtain the first success :
P(Y = 1) = p
The event "you obtain the first success after k times" means "no success for k-1 times" and "sucess at the k-th try". Therefore :
P(Y = k) = p*(1-p)^(k-1)
Finally if you want P(Y = k) = 0.8, just solve for k this equation.