Before you can start getting paid, you'll need to reach the payment threshold. This varies depending on your currency. In the US, the payment threshold is $100. This means you'll need to earn $100 before you can start collecting any money. If you've hit your payment threshold, you'll be paid around the 21st of every month. If you didn't meet the threshold, that money will be rolled over into next month's amount.
Typically only offered to large YouTube channels with a wide audience (although not exclusively), another big way to earn cash through YouTube is to get sponsored deals with companies that will pay you to promote or mention their products in your videos. You can earn money this way either as a lump sum of cash the company will pay you for the deal, clicks on the company's link, or on a per-view basis.
YouTube earns advertising revenue from Google AdSense, a program which targets ads according to site content and audience. The vast majority of its videos are free to view, but there are exceptions, including subscription-based premium channels, film rentals, as well as YouTube Premium, a subscription service offering ad-free access to the website and access to exclusive content made in partnership with existing users.