Looking for advice on on going pro

I've said for years that I didn't think I wanted to try to publish any original fiction, but I've changed my mind about that in the last year. For a long time it didn't seem that the kinds of things I'd want to write would get published, but that's clearly no longer the case. I know so many people in fandom who've published m/m romance and erotica, and so I've been thinking about whether or not I want to do this too.

And yes, after stewing on it a year, I think I do. I have an idea for a book and have been thinking about it obsessively, and in another month I will have time to sit down and start working seriously on it. It's an idea I'm really excited about, as excited as I usually am for fanfic, and I feel like I need to go with my energy on this.

So I'm looking for some advice on how to proceed. I know many of you have experience with this, and I would appreciate any tips you could give me. I'm not sure what to do first. Do I write the book and then shop it around, or do I write a synopsis first and use that to look for an agent/publisher/whatever? I am a complete n00b in this area.

Any resources you could recommend or advice you might have would be greatly appreciated!