Once you've deleted your account, you won't be able to use it anymore - obviously! But seriously, all your messages, contacts, and everything will be gone. So, if you have anything important, make sure to back it up before you delete the account.
The whole process is actually pretty straightforward, and you'll be done in a few minutes. Just remember that it's permanent, so think twice before you do it!
How to delete your Telegram account the right way