Banners are visual displays that come in industry standard sizing. For example, a standard banner is 468X60 and a leaderboard is 728X90.
Banners are usually sold by impression, with CPM being the cost per 1,000 impressions. Some vendors will sell per click. With this model, the vendor continues to serve impressions until the quantity of clicks you ordered is fulfilled.
There are a number of platforms that allow you to create your own banners. Sites like Upwork and Fiverr have designers to do all the work for you.
Your banners should be unique and have a key point to emphasize. Banners are not just for brand awareness. People still click a banner that piques their interest.
Test banners to find the best performer. When testing, only change one variable at a time, such as background color or text. For example, if your banner has a blue background, run another with the same images and texts but use a yellow background.
Use a third party service to track your ads. Continue to run the banner with the best click rate, then test it with another design to see if you can do even better with your click conversions.
There are vast advertising resources to advertise your banner. The programs listed below are just a small sampling of what is available. Affiliate link notice.
Banner ad exchanges and programs
Banner creation programs