Go to the Tickets section, "Create & edit" and click on the + to add a new ticket type.
Choose "Paid ticket". You are then asked to enter currency and price - besides ticket name and total quantity of that ticket type.
If you click on "Options" you will have other settings for the ticket type available, for example if you want to include tax in the ticket price. Check the relevant articles at the bottom.
Remember to activate Paid Tickets
To activate paid tickets and receive the payments, you need to connect a payment service (Stripe or PayPal). This is done under "Payment & tax" in the Tickets section. See article at the bottom.