Firstly, take a deep breath. You have a month from the date of the decision to get your appeal in. I'm guessing your letter may have been caught in the Christmas post so you may not have a full month left, but you've almost certainly got a while to sort yourself out.
If there's a form GL24 with the leatter, put it somewhere safe, or get one from a Jobcentre or the Gov website. You must use that to appeal on.
Then, if you can, try to get some face to face advice from a welfare rights adviser.
Meanwhile, look at the guide to DLA appeals in the Help for Claimants part of the site. It takes you through it step by step.
Good luck.