Most newbies only build defense at their entrance. You can use that to your advantage. When you begin to make your first zealots, have a group of about 4-6 ready and then attack. But instead of attacking the defense, just have them walk right by since zealots can take a lot of damage. This may not work so well against photon cannons if there is a lot of them. The speed upgrade is good too but you probably wouldn't have it this early in the game. When you're inside start to attack the resource workers or the nexus/CC/hatchery. Just remember to make more zealots or whatever while you're attacking. If they do manage to somehow live, you have slowed them down and shouldn't have too much of a problem.
This just doesn't only work on newbies, some people just build defense at the edges of their base and not on the inside.

CreepColony's Opinion: Also works against those zerg players who build a hatchery at their entrance.