If you are looking to farm act 1 keys to make your infernal machine you should check out this video. Probably the fastest way to get this done is going to be to grab some friends to form a multiplayer game. Public games can work too but sometimes finding people that are geared appropriately and are on the same page as you can be tough.
Head over to The Festering Woods to get your 3 - 5 stacks there. The Crypt of the Ancients and The Warriors Rest are always present on The Festering Woods map, as well you can always find either events or wandering champ/elite packs outside in the woods. Once you've done that, if you don't already have your 5 stacks of nephalem valor (NV) which will be required to have a chance at a key drop, head over to Leoric's Manor. You can count on getting 1 - 3 stacks between Leorics Manor and Leorics Manor Courtyard. These are small, fast maps to get to and clear.
If you're really unlucky and still don't have your 5 stack, head to the Cemetery. Sometimes you can find a pack outside around the waypoint port in. If not, simply head into a crypt as there are always packs in the crypts. Alternatively you could also go straight to the Fields of Misery where Odeg the Keywarden can be found.
If you do go to the Fields of Misery before you have 5 NV stacks, make sure you find pack(s) to finish off your 5 stack before you kill Odeg. Don't worry if you run into the keywarden too early. Just run away. You don't have to kill it before you have a 5 stack.