Cash App wouldn't be my first choice as a place to store my Bitcoin, but you can definitely use it. You should see "View Bitcoin Address" under "Deposit Bitcoin". If you click that, you'll see a long string of letters and numbers. That's the address you need to copy into your payment settings on whichever site you're using (definitely copy and paste--you don't want any typos!) And that's pretty much all it takes.
Once you get the deposit, you can sell it for USD and use your account as you normally would.