Like the previous two games in the series, Fallout 3 has a recruitable canine companion named Dogmeat. He's a bloodthirsty little bugger and can be a huge help in the early stages but the game doesn't exactly throw him into your lap. In fact, you could go through the entire game without finding him if you don't know where to look.
You can recruit Dogmeat pretty much right after you escape Vault 101, but it'll take a little legwork. First, go to the town of Megaton. Follow the ramp above the clinic to a store called Craterside Supply. The shopkeeper Moira is working on a Vault Dweller's Survival Guide and needs you to do some research. The first chapter has three quests that you can perform in any order you choose. Pick the "Minefield" quest. She'll mark that town on your map.
Walk up to Minefield. You'll have to cross a river at some point so just watch out for mirelurks - they're giant, heavily armored crabs who can be a bit tough with underdeveloped combat skills. If you run afoul of any, shoot them in the face (their only weak spot) and run away sobbing. On the way up to Minefield, you might encounter a giant junkyard called simply "Scrapyard". That's where we want to go. It's south and slightly east of Minefield but not too far away. It's pretty massive so you shouldn't have any problems spotting it.
On the northern side of the junkyard, you'll come across a dog fighting a few raiders. The raiders are pretty weak so the dog should be able to beat them up by himself but you can shoot off a couple rounds at them if it'll make you feel righteous. As you may have guessed, the dog is Dogmeat. Talk to him after the raiders are dead and you can get him to tag along with you on your journey through post-apocalyptia.
If he dies in combat, he's dead for good so take care of him. You can open a dialogue with him and tell him to wait somewhere if you've got a not-so-dog-friendly mission to do. Taking him through the landmines back in Minefield, for example, probably wouldn't be a good idea. It's also worth noting that Dogmeat doesn't count toward the game's one companion limit. Thus, you can recruit Dogmeat and still be able to grab another companion with opposable thumbs.
