Right After you leave Vault 101 go to the city eastish called Megaton.

When you go there talk to Lucas Sims and he'll ask you to disarm the bomb in the center of the town.

Go up to the bomb and choose rig to blow.

After you watch the shoe go in to the wreckage and loot everything and sell it to local markets or keep the stuff for future use.

Note: this will give you bad karma.