Making position for banner

Hi there
I`m need to make a position to the right of content to be 300px wide to hold my banners (300 x 250)
How can I do this...?

Also what does the numbers under dimensions in template backend stand for:


Left column width 20
Left 2 column width 50
Right column width 28
Right 2 column width 54
Inset 1 column width 24
Inset 2 column width 15