Create new admin - please help

Why not just create a new user and update their status in the profile page?

Anyway, it's just a simple SQL query
UPDATE prefixusers set g_id = 1 where id = admin_id

replace prefix with the table prefix (or remove it if you have none)
replace admin_id with the id of the person you want to make admin

yes, because you're missing the table name
Like I said, replace prefix with the table prefix
So if the prefix for your tables is foo_ you would put foo_users as your table

Meh, my mistake, use group_id instead of g_id

So, like this:
UPDATE users SET group_id =1 WHERE id =2