To accept online payments with Stripe.com and Paypal.com, click on the Gear Icon at the top right of the screen to go to the Settings page.
On the Settings page, click on "Setup & Manage Online Payments" and from that page, you will see the options to enable both payment gateways.
Stripe can be used for recurring payments while PayPal can only used for one-time payments.
Each option also gives you the option to add fees to each transaction, should you choose to do so. You can also disable the fee on the customer's end so they aren't charged if paying with Stripe or PayPal.
Please note: You can only set up online payments from a desktop computer, not the app.