Stripe has a great online support setup. You can check it out here: https://support.stripe.com/. Can't find what you're looking for in their online database? Write them an email and they'll get back to you pretty quickly with an answer: https://support.stripe.com/email.
Need help with your Square account? You can contact them directly! Once you've signed up for a Square account, you'll get a customer code that can be used to give them a ring at 1-855-700-6000 (M-F 6am-6pm, PDT). They're also happy to chat via email once you're logged in. This option can be accessed right from their support center (https://squareup.com/help/us/en/).
If you're not quite ready to sign up, you can reach out to their Sales line instead! That number is (800)474-3398 and they're available from 6am-5pm, PDT, Monday-Friday.