You can get bitcoins by mining them. This is currently done by special mining hardware. You can mine alone or as a part of a larger mining pool. Before that you should buy this relatively expensive hardware (for example AntMiner S9).

The usual way to get bitcoins is buying them with regular (so called fiat) currencies (like USD, EUR...) through bitcoin exchanges. More on the process: https://www.investopedia.com/terms/b/bitcoin-exchange.asp. To get more details you should check on particular exchanges mentioned in the article (like for example: https://www.kraken.com/).