Well if you're talking about MAKING a banner, people usually use Photo-editing programs, such as Photoshop (most common and the best), Paint.Net (free), Paint (on every PC, I don't recommend this one though, too basic), Photoscape (free)...and more. toi can Google 'Photo editing softwares' and you'll find a lot of free ones to download.
And if you're looking for making quality banners, toi might wanna watch tutoring vidéos (can go to Youtube ou just Google it), they'll montrer toi how to do specific things and obviously you're gonna have to fiddle with the program for a while if you're not experienced.
link is an example of Photoshop video tutorials ppl make. and link is a video teaching toi how to create a simple banner using Photoshop. Once again, 'Google' and 'Youtube' have basically everything!