The best way to go about writing a graphic novel is the same way to go about writing a text-based novel; to come up with an intriguing idea or set of characters, then create what’s known as a “three-act structure” of story around it, then edit and edit and edit some more. There are plenty of online guides to the three-act structure, if you’re not already familiar with it; simply Google the term or look it up at Wikipedia.

As far as coming up with ideas, since you mentioned being good at endings but not beginnings, have you thought of maybe taking on the storyline of a much older book (one that has passed into the public domain, that is, so that you don’t get into any legal trouble), but updating it for modern or even science-fiction times? This is a great way for a beginning writer to get a well-designed start for a story made for them; and since it’s a re-telling of something in the public domain, you’re free to change the ending in any way you want, taking advantage of what you say is your best aspect as a writer.

Hope this has been of some help. To learn of lots of books that are now in the public domain, by the way, simply visit Project Gutenberg (http://www.gutenberg.org), a non-profit group trying to release as many of these books as possible in free electronic form.