If you are graphically challenged, that's OK. There are lots of people who ARE good at graphics, and for a few bucks they'll gladly help you create what you need.
Head on over to the magical resources that is Fiverr:
and signup for an account. They accept paypal and bitcoin to pay for the services listed.
Search for 'banners' and look for the guys that have lots of reviews. There are a handful of good ones there. Look at the samples, and if it matches the style you are going for, 'buy the gig'.
After you pay, there will be an automated message asking for the details of your banner. What colors, what size, etc.
Here are some of the most common sizes for your banner, for a full list goto: https://en.wikipedia.org/wiki/Web_banner
728 x 90
468 x 60
300 x 250
160 x 600
120 x 600
Another place you can get inexpensive quality banners made is:
Now that you have some banners, you want to be able to TRACK to see which ones are working the best click here to see how to TRACK your ads