Hey Neil - welcome to the board! Hope you like it here, and that we can help some.
You should probably also think about which publishers to approach with what material. For social realism, someone like Top Shelf may be your best shot. I think that apart from the big two, though, pitching only really makes sense when you’re already coupled with an artist, so that really is your first step, yeah. I think pitching to the big two doesn’t make a lot of sense at the moment unless you’re an established writer - approaching them with a brilliant pitch as a no-name and becoming the next Alan Moore or whoever has, I think, gone from a million-in-one chance to pretty much being completely out of the question, these days.
As for the pitch itself, I like to point people to Warren Ellis’ Come in Alone column about that (which is a bit out-of-date at this point, but still pretty good in general principle, I think):
It’s part of his series on comics writing in general, all of which is well worth reading for someone looking to write comics.
Um, a word of warning, maybe, if you haven’t been observing the field a lot in recent years. There are a lot of people who want to write comic books. Like, an ocean of them. Most of them aren’t serious competition because they come from an angle of, “Oh, I know how to write Spider-Man the right way, I’ve got the ideas all figured out”, but without ever having bothered to do some actual writing. But there’s also a lot of good writers out there, in what is still a pretty narrow field. If you’re looking to make a living any time soon, your chances are better in traditional print, television, or whatever.
That being said, if you’ve got an actual 22-pages comic as a calling card, that puts you way ahead of most people. And as for finding artists, Gar is right - this isn’t a bad place to go looking, but it’s probably best to be specific and approach artists directly. You could also create a thread in which you describe the project and see if anyone’s interested, of course.