okay heres how this works
advertise on a site like webhostingtalk.com that you make banners for the modest price of 5 dollars or something. say you are building your portfolio so you don't have much to show at this time and that you don't accept payment until the customer is %100 satisfied. then when someone wants your services you ask them what they want the banner to say then find a banner on google images like this one:
load it into photoshop and just change the wording/images to suit your customers needs, e.g.
thats just a quick 5 minute job i did and its a really simple example but you guys get the idea. this way anyone can make web banners even if you aren't at all creative.