Sign up for Stripe at https://stripe.com/. If you do not have a corporation yet, do it as an individual or sole proprietor. You can change this after. We just need to set this up now so we can send invoices and collect payments.

(Optional)

If you want to create a US corporation (even if you don't live in the USA), I would recommend using Stripe Atlas at https://stripe.com/atlas. This costs $500 (at the time of writing), and it will set up a corporation for you in Delaware USA. You will get an EIN which is needed to open a bank account.

You pay, tell them the name, the shareholders (usually, just yourself), and they will do the rest. This is a great option for both Americans and non-Americans.