First thing to do is revise the book as well as you can (if you haven't done so already), and make it the very best that it can be.
Then write a synopsis for it (one or two pages, single-spaced). This doesn't have to be written in the voice of the book, it just has to cover the main plot of the book. You won't have room to cover all the subplots, so don't worry about them: just make sure you put the main story down including the ending.
Then write a covering letter to go send out to agents. It has to grab their attention and make them interested in your book so this bit has GOT to be good.
(Nicola Morgan has written a couple of e-books on the subject of submitting which I think are very good: Write A Great Synopsis, and Dear Agent.)
Then research the agents you want to submit to. Look for agents who have made good sales to good publishers, for books in genres similar to yours. You can use Google but it's also helpful to go into bookshops and look in acknowledgements, etc, to find out who reps who.
When you have your list send your submissions out, five or so at a time. Follow the agents' guidelines to the letter (they'll be on the various agency websites). Expect to wait a few months for responses. As rejections come in, send a few more out. If you get lots of form rejections (rejections which are not personalised in any way) then there's probably a problem with your query, and you need to rework it. If you get lots of full requests and then no acceptance it's probable that your ms needs more work.
Try to remember that a rejection of the book is not a personal judgement on you, even though that's very hard to do. And good luck!