You can add a product right in the app. Open the menu, tap Products, then tap + (or Add Product if your list is empty). Enter a name and an Amount and save. It goes straight into your Stripe account. Teams staff logins can't add products; the account owner can.
For recurring products, or more than one price per product, start at dashboard.stripe.com/products/create. Products you add there show up in the app too. Guide: How do I add multiple Prices to a product?
You can also set a product to Recurring, which lets you offer subscriptions on different cadences: daily, weekly, monthly, yearly, and so on.