First, of all How did you get Banned? Well, Ask yourself these Questions.
How Long is my Ban? How Did I Get Banned?
If Your Ban is One Day. Dont Bother Looking for help, just wait it out.
If you got a 7 Day or 14 Day Ban, Try contacting the ROBLOX Appeal's Email: email@example.com
And email them, For example try telling them why you did or didn't do what you did, (Such as you made a mistake)
By the way, make sure to "Give Roblox a good reason of why you didnt deserve to get your account banned" if your planning on Emailing the Appeal for ROBLOX.