Bitcoin mining is used to approve the financial transactions in the Bitcoin market, to make transfers and to produce new Bitcoin tokens. Bitcoin Mining that workes on a blockchain base and a decentralized system rewards users who produce fastest blocks and validate operations. Whenever a new Bitcoin token is produced, the difficulty levels of the blocks are also increasing, and therefore computers with very powerful processors are required for mining.
There are many platforms where you can buy Bitcoin. These platforms offer a wide range of options to users from credit card to the money order.
The bitcoins you bought will store in your wallet that you use. When you decide to sell, you can exchange your Bitcoin tokens stored in your wallet to USD or other cryptocurrencies.