Congratulations! You've actually written, and finished, a book. Lots of people dream of being a writer, and a few of those will sit down and write something, but only a handful will complete an entire manuscript. Before you do anything else, relish this moment of accomplishment. You've done the equivalent of climbing Kilamanjaro. Good for you!
So what's the very first thing you should do before you start sending that beautiful book of yours to potential agents?
HIRE A COPY EDITOR.
I cannot stress this enough: hire a copy editor. Even if you have a PHD in Literature or an MA in English, hire a copy editor. Even if you are the best speller in your entire State and you've got the spelling bee awards to prove it, hire a copy editor. Do not send your lovely novel to agents full of typos. It's like showing up for a job interview wearing a dirty blouse.
I'm assuming you've been passing chapters of your novel to friends and family for feedback, and I'm also assuming those friends and family have been giving you good writing advice (if not, you need to hire a different editor to work on plot and character development). But it shocks me when I see how many writers do not hire a copy editor for a final check of their manuscript. I get plenty of submissions ruined by spelling and grammar errors. Know what I do? Toss it. I am way too busy to work with an author who can't take the time to fix spelling errors. It shows me that they do not take their writing career seriously.
I hire a copy editor for my own writing, as well as for the books I publish via Medusa's Muse (although I don't hire her to edit my blog, which as you see could use the help).
The problem is that none of us can see our own mistakes; we are blinded by the words on the page (what I call writer's goggles. They work a lot like beer goggles). Everyone has this problem, and anyone who thinks they don't are fools. I'll bet you twenty dollars that even President Obama hires a copy editor for his written work.
Where do you find a good copy editor to polish your masterpiece? I'll explain that next time.