You need to know which egg to incubate.
One of the more exercise-focused aspects of Pokémon Go is hatching eggs. Each egg has a distance meter next to it, and when you put that egg in an incubator it becomes active. When you walk the distance marked on the egg while playing the game, the egg will hatch and you'll get a brand new Pokémon with a bunch of Pokémon Candy for that creature.
The difficult thing here isn't the walking, it's choosing which egg to incubate next. You only get one incubator with unlimited uses, and every other incubator — whether purchased or received from a level up — has a three use limit. The best way to choose which eggs go in which incubator is to know which Pokémon are in the egg to begin with. While there's no what to know exactly which creature is in your specific egg, here's a list of Pokémon and the egg types they show up in.
And that's it! From here you can plan out which eggs would be best for you to hatch right now. For example, focusing on 2km eggs is great if you are hoping to quickly level up a starter Pokémon, and 10km eggs are great if you want something unique. The only uncertainty with these eggs is Candy.
The amount of Candy you get from each Egg is random, somewhere between 5 and 35 Candies. That could mean you get an Eevee with everything you need to evolve it on the spot, or it could mean you get a Pidgey with 5 Candy that makes you sad on the inside. Either way, hatch as many as you can!