This guide will tell you how to get access into Saffron City:
Basically, you head south of Cerulean and encounter a guard that requires a sip of tea to allow you to pass. To find the tea, you head to the Celadon Condominiums and find Brock who will give you the requested tea. Then, all you have to do is give the tea to the guard blocking your way to Saffron City and he'll allow you to pass freely.
