if a cannons up already (you should always see it though like Zawmb said the 2nd ovie should be in your natural) you usually either have to make a spine and push your creep to the ledge to take out the pylon/cannon from the high ground and move down, or you can 1base roach and kill it (not as economical) although you might win if he's dumb enough to not make defense at home. If you see the cannon up tho I'm pretty sure you're suppose to take the next expansion.