First and foremost, principally anyone can be miner. Miners are needed because of the nature of the decentralized network where they have noauthority to delegate tasks and the cryptocurrency needs some kind of system to prevent any form of network abuse. For instance, a person may createthousands of peers and spread forged transactions. It will disrupt the system immediately.
In order for you to be a miner, you would need to solve a cryptologic puzzle which is a set of very complex mathematical questions set by Satoshi Nakamato himself. If you successfully solved the puzzle, as a miner you can build a block and add it to the blockchain.
The miner is also given permission to add a crypocurrency transaction to the system which automatically grants him a specific number of bitcoins. This is the only way to create valid bitcoins. Bitcoins can only be generated if a miner can solve a cryptographic puzzle. The level of difficulty increases with the amount of computer power the miners invest.