Monero works on the same principle as Bitcoin i.e. Proof of Work.
What is proof of work?
All miners lend their computation power to the Bitcoin network and verify transactions. These verified transactions are clubbed into a block. On every block mined, Bitcoins or Monero is awarded depending on the Blockchain you are working on. For the purpose of earning via mining, you need to understand this that the probability of you mining a block is proportional to your contribution to the whole network hash rate.
Monero is a Proof of Work cryptocurrency that can be miner with computational power from a CPU or GPU. There are currently no ASICs for Monero, which means that anyone with a computer can mine it.
On an average a block is mined in 2 minutes, i.e. 720 blocks are mined in a day
Total network hashrate is approx 77.5 Mh/s
Mining reward is approximately is less than 7.5XMR/block currently
Using the formula n = (Network Hashrate) / (720 * Avg Block Reward):
n = 14444.4 H/s, or 14.44 kH/s to mine 1 XMR per day.
You would need approximately 60 GTX 750 Ti's at 250 H/s each, OR approximately 25RX 470's at 600 H/s each, OR approximately 20 RX 480's at 750 H/s each to mine 1XMR every day.