If an event requires paid tickets, you'll see a Get tickets button on the event page. It is not possible to RSVP without purchasing a ticket on ticketed events.
Tap Get tickets and select your ticket type and quantity (you can purchase up to 10 tickets for any given event)
Tap Next. Add or confirm your contact info, and enter a promo code if you've received one.
Tap Checkout. Answer any questions from the host (if applicable). Enter your payment details manually, scan your credit or debit card, or use Apple Pay. Tap Pay. You'll see a "You're going!" confirmation screen with your QR code ticket.
Your ticket will live in the event page under View ticket. You can also add it to your Apple Wallet.
Click Done to exit the ticket view.