Go to Account Settings → Connected Services and click the “Connect to Stripe” button. Follow the prompts to either create or connect a Stripe account.
Once you have connected your Stripe account you will have two payment accounts created, one in “Live” mode and the other not. You can use the account that is not in live mode to test your submissions. See https://stripe.com/docs/testing#cards for a list of test credit cards.
You can choose which account you’d like to use on a a form by going to Configure → Payments and choosing the appropriate account.
There are restrictions around multiple accounts depending on the plan you are on. See Pricing for more information.