To mine bitcoin you need to have a computing machine that can solve complex math puzzle. Mining company runs some equipment that can solve those math problem. Actually in every 10 minutes bitcoin creates new block. The miner who can solve first the problem can solve the get 12.5 rewards. So it is one type of first solve first reward. If you run mining from your home pc, there is a little chance to get those block rewards.
Mining pools set up those machine and rent those equipment to the users. There are some reputed company in the market now. You can but any package to mine bitcoin or any other coin. They splits the 12.5 btc rewards among the users.
Currently mining pools are not profitable, so before buying any package consider that. But it can be profitable if btc price increases.