Once you are in the Wasteland, head to the town of Megaton. Speak with Moriarity in the saloon at the top/back of the town. You will need to get info about your father's whereabouts, as usual there are a few different options to procede.
Once you find that he went to Galaxy News Radio, head there and aid the Brotherhood soldiers in killing all the mutants, including your first Super Mutant Behemoth. After they are all dead you will be able to use the intercom on the GNR building to enter. Speak with Three Dog upstairs to find out your father has already moved on.
See Following in His Footsteps for more specific info.