You will want to get a literary agent. Most publishing houses won't read unsolicited manuscripts unless they come from a reputable agent. Not only that, but an agent will negotiate the terms of your contract for you so that you can keep as many of your rights to the book as possible.
I really like the current issue of the "Guide to Literary Agents". It is from the folks at Writer's Market, who do an excellent job is creating resources for authors to start their research of agents. I'd also recommend their book "Formatting & Submitting your Manuscript".
As for finding out if they're reliable, and excellent resource is the website Preditors and Editors-- they are a good jumping off point to start your quest!
If you have written any poetry or short stories, you should try submitting them to journals or magazines that are appropriate (Writer's Market can help you find some). It helps if you have proven that your writing can sell! Agents like to know that you are a commodity people are willing to spend money on.
If you haven't read them yet, check out these books for help in your writing:
1) "On Writing Well" by William Zinsser-- This is an excellent reference for any kind of writing. It will make your emails, essays, stories, and other writing clearer. I recommend this to everyone who wants to improve their writing skills.
2) "Bird By Bird: Some Instructions on Writing and Life" by Anne Lamott-- This is slightly more geared towards fiction writing, but can be helpful when you are attacking ANY large project in your life. Definitely a must for creative writers, students, or people who are looking for an entertaining book on writing.
3) "Writing Down the Bones" by Natalie Goldberg. This targets creative writing, and focuses on the practice of writing. I write a lot of poetry and am working on a novel, and this is incredibly motivational to me.
Do your homework, and find out exactly what they want in a submission package or a query letter. Then submit over and over until you finally find an agent who accepts you. You will face a lot of rejection, but keep going!