How do you get someone to follow you in fallout 3

Talk to them and there should be some dialogue asking them to follow you.

e.g. Dogmeat can be told to wait on the spot or follow you or sent back to vault 101, though when ordered to Vault 101 by the player from the Citadel, sometimes he will just disappear and you will never get the 'Dogmeat is waiting at Vault 101' message.