To advertise your affiliates banners or Google ads in your WordPress sidebar is very simple. First, you will login to your WordPress administration area, then click Appearance in the left side menu. Next, your going to click on Widgets. Once you've done that, you simply drag a new TEXT widget to your side bar, click the arrow located on the top right of the widget box, then copy and paste the HTML or Javascript snippet from your advertiser into that box. You don't have to give it a title, but you can if you'd like. Click save to make your changes.
Note: for this to work, you must have a sidebar.php file located in your themes root folder, which most usually do.