You can also create a colony by right-clicking on the body on the system map and selecting "create a colony". Or just by unloading colonists or mines or the like.
This is absolutely essential if you want to mine an asteroid/comet. They won't mine until there's a population on the asteroid/comet, even if it's a population of 0.0 they will still mine out minerals there. On the map, zoom to the asteroid/comet to be mined, right click on it, scroll down to the last option, and then click 'create population' and there you have it. Once that's done simply have the miner go into 'extended orbit' around the asteroid/comet. And don't forget to put a mass driver on the asteroid/comet to be mined
AND MORE IMPORTANTLY ONE ON YOUR HOME PLANET. But if you feel like using a ship to transport the minerals from the mine you can use a freighter if you plan on doing it manually via ship.
By the way, for an easier time, next game, just tick off the overhaul and maintenance button so you don't need to worry about that.