i was sent this as a retrospring ask but i was writing so much that i didn't want to lose the text. so now it's a long post. anyway the way i got in started with an event that i don't think exists anymore (an agent pitching hashtag on twitter) but the traditional ways still exist and i know enough about them to sketch it out.
but before we talk about actually getting into the industry, do you have a complete outline written that tells potential agents and editors exactly what happens in your story? do you have a sample of 8-10 COMPLETE pages from (the start of) that story ready to go? do you have an idea of where in the market your work fits? middle grade, YA? sci-fi? romance? fantasy? nothing else after this point is worth reading or thinking about until you have those things prepared. go do them and come back.
okay are they done. alright. you are going to have to find an agent. do not try to approach traditional publishers without an agent. honestly you should still be wary of approaching small pubs without an agent. an ideal agent know the editors and publishers who would be the best fit for you, and will be the one to put your work in front of them. they'll also be the one to fight on your behalf for a good deal (because they don't get paid unless you do) and will handle any awkward bits of business that you're not good at (like if your editor is dragging their feet getting materials to you, your agent is the one who can yell at them about it so you don't have to).
if you want to work in traditional publishing, you must have an agent.
but you have to get one first. and you have to find them. probably the quickest way to find a graphic novel agent is to go look at your favorite comic artists' twitter profiles and see who's repping them, a lot of people put their @ in the bio. you can also just look for websites that collect literary agents. look at what they've previously got published, and what they have in their wishlist of things they want to/like to publish. it's okay if you're not perfectly 1:1 with these things, but ideally you're close. pick three-five of these agents to start with. now you need to query them.
querying an agent is a fancy way of saying submitting your work to them to see if they like it enough to represent it. every agent/agency has their own way of receiving your work samples, there's no set standard between agents, especially not for graphic novels. this is why i told you to get 10 pages and a full outline ready, to make absolutely sure you have enough of what they would want to see. you're going to send in whatever material they ask for (and please god to the best of your ability fOLLOW THEIR INSTRUCTIONS). and then you wait. you will wait for weeks, and you may wait for weeks just to get a rejection.
but good news! after five weeks, two of the five agents you queried responded and are interested in your work! you are the belle of the ball. you are also now somebody in a hiring position. you need to remember this fact especially: you are hiring someone. your instinct is going to be to defer to them and their insider knowledge and just be honored anyone would deign to look at your work. resist this instinct. be strong, be professional, remember that you are hiring them. don't just go with them because they want you and they're nice! go with them because you think they understand what you're trying to do with your work, and that they'll be the best person to represent it.
when i was querying my graphic novel, i spoke with three agents who were all completely competent, all interested in my work, and all had different ideas of what to do with it. one of them thought my story would be easier to sell with a bunch of changes to make it more middle-grade, when i wanted to publish it as a YA. and she might have been right, but that wasn't what i wanted to do. one of them was very nice, professional, honestly there was nothing wrong with her at all, but the agency she worked for felt too big and too impersonal for me. the agent i went with, and who i adore if she ever reads this, was a perfect fit because she understood exactly what i wanted to do with my book, had an energy i could gel with when we spoke, and works with a smaller but clearly successful agency that also does a lot of romance--which was good for me as somebody wanting to eventually break into that space as well. a good fit for me and my needs as an author!
and then it's a matter of selling the book to an editor which, if you've managed to get an agent, will go differently for every book. your agent may still suggest changes to your outline (they will usually tell you what changes they have in mind in the initial phone call, but be sure to ask!) or they might ask for new samples. this is normal and fine, and you should do your best to get them that material. they know what they need and where it should go!
and then...... they'll send it to editors they think would like it! and you'll wait forever again. but where it goes from there is going to be different for everyone and by this point it's just "do the work and hope for the best". the hard part of getting something out there is done, if you made it this far. if an editor wants your book, further instructions will follow.
and that's the bare bones of you how get into making graphic novels professionally.