John Smithers' advice is good, but I'd add a few details (and leave it for longer than three weeks!)
Before you put the MS away, make a first pass at your query letter, as well. This is good because the query needs some time away from your eyes just like the MS does, and because writing the query can really help you figure out what the book has going for it, and what it's lacking.
So, after the query and MS have rested and you've forgotten about them, come back with fresh eyes and look at big stuff, first. Write your synopsis now, I'd say, and look for pacing issues (too many chapters with no action, or too much action all the time, etc.) Make sure the book you have matches the book you wrote about in your query, and if it doesn't, change it. Check for character consistency, development, etc. Make sure your plot structure is solid (rising action, climax, satisfying conclusion, etc.). There are entire books on editing out there, (Self-Editing for Fiction Writers by Browne and King is one good one) so I can't give you the whole story, here, but you get the idea. Obviously you also need to look for style issues, and typos and mechanical errors.
Then send the book to betas. While you're waiting for them, you can start looking at your publishing option. Steven Drennan will probably be by shortly to recommend that you self-publish, but you may also want to consider getting an agent and going through a publisher. You can find suitable agents by finding the agents who represent your favourite books in the same genre as your MS, or by using sites like Query Tracker. You may want to post your query at the AW Water Cooler or other sites that offer query critiques, because writing queries is an art in itself.
Once you've got responses from your betas, look them over with an accepting but critical eye. If more than one person has a problem with a certain aspect of your book, take a hard look at it. Even if only one person finds something, give it a good look. But don't let your betas rewrite your book for you. You're still the author, this is still your book, and you have the final decision on how you want it to go. If you make significant changes, I'd recommend letting the MS sit for another chunk of time before coming back and deciding whether the changes work. Make sure you keep copies of all versions of your work so you can revert as necessary.
When you've finally got the MS in the best condition possible, assuming you're not self-publishing, send out a batch of 5-10 queries to agents. Don't query them all at once, because sometimes your query won't be as good as you think it is, and you don't want to burn bridges (once you've queried, you can't re-query the same agent with the same project, generally). You may want to include your first choice agent and a few mid-level guys in the first batch, leaving your second choice agent out of it; that way, if you decide your query wasn't strong, you can send your revised query to your second choice agent and still have a good chance at a good outcome).
If you can't get a bite on an agent (and I'm talking about after a couple hundred submissions), you could try to submit directly to the publishers who accept direct queries, but there aren't many. You could also look at self-publishing. There are lots of how-tos out there on how to do a good job, but be prepared to invest some significant cash in cover art and professional editing. You may also want to consider writing under a pseudonym; if you couldn't interest any agents, and if you're serious about writing more, you need to seriously consider that this effort really isn't good enough to be published. (It's also possible that it's great quality but in a weird niche, or an over-crowded genre, or something else). But once you've published under your name, your book's out there, and if you later come to realize that it's poor quality, it's too late to save your name.
Some of this is genre specific, of course. If you're writing erotic romance, you could jump straight to the e-pubs without an agent.
TL;DR version: You've only just begun! But congrats on an important first step.