Well it is very simple

U not the only one i started just like you and i still don't know much of the stuff :P well the only way to learn is to make mistakes (that's what i have learnt) Just keep it up and everything will b aryte man. I will try to give u some guidelines and hope this will help
FIRST & MOST IMPORTANT : MAKE "BACKUP" before you do anything else
Here are the steps to delete that useid from your database.
Login to your "cpanel"
Click on "phpMyAdmin"
Select your "database name" from left side
Type "Customprofile" on the left side where currently is should say "filter tables by name" remove it and seach for "customprofile"
Click on "customprofile" table and now look at the right side.
look at the use id table and see if u can find "-1" in that table and if u do just put a check mark from the left and click on delete or simply just click on delete.
Attaching pics also to make it a lil more helpful. (hope so)
If this helps let me know. "please make sure to create a backup before u make any changes"
Thanks