Sidebar modification

Hi i am new to wordpress and i want to modify my sidebar.

now it is sth like this:
category 1
category 2
category 3
and i want to make it show how many news are posted in the category

category 1(15)
category 2(4)
category 3(9)

where is defined the function that renders the sidebar ?

oh and how to move the sidebar on the left part of the site?