One of the many rewards from Pokestops include eggs. These come in three standard varieties; 2km, 5km and 10km. While special eggs are sometimes available, these three are your regular drops. You have one infinite incubator in your item bag so always make sure you are using that as you go for walks. You’ll find your eggs by swiping right from your Pokemon collection screen.
More incubators come in two different flavours. Standard and Super. Super Incubators will reduce the distance necessary to walk but are more expensive to buy in the shop. Regardless of which type they are, other incubators sadly can only be used three times before they break so make sure to put lower distance eggs in your infinite incubator and save higher distances for breakable ones. Good old maths.