Counting User Post Count ?

I didn't think this was going to be so tricky. Basically I have two tables in my mysql database (user, comments). Each table has the users id. I simply just want to count how many posts/comments each user has and then place it in the user table, in a field called 'postnumber'. Can anyone guide me in the right direction on how I can achieve this ? I've searched the net but haven't found any useful information.

Thanks in Advance !!