Hatching EggsIf you haven't heard or realized it already, a great deal of your pokedex and earning xp will come from hatching eggs. Eggs can be found only at PokeStops, and they can range in hatching distance from 2km to 5km to even 10km. (After a recent update, all 2km Eggs are colored Green, all 5km Eggs are colored Orange, and all 10km Eggs are colored Purple.) Only 1st Stage Evolutions of a given Pokemon Evolution Chain can be found in Eggs - some Pokemon can't be found in Eggs at all - and each species is set to a given egg Distance Group (Ex: bulbasaur can only come out of 5km Eggs, not 2km or 10km Eggs.)
walking is the preferred method for hatching, but skateboarding, biking and driving under 10-15mph can also get the trick done. However, the app must be kept open in order for the distance to be recorded, and one must actually travel distance instead of just walking around a given house or building.
hatching an egg, as mentioned, will also help your trainer gain loads of xp. You will earn x100 Experience for the distance of the Egg that hatches (2km = 200 XP, 5km = 500 XP, 10km = 1000 XP). You will also gain much more Pokemon Candy through hatching rather than capturing, anywhere from 6-30 Candy depending on the egg and what hatches (compared to only 3 Candy for a Wild Pokemon Capture). The more distance an Egg requires for hatching, the more Pokemon Candy will be given.