First of all, CONGRATULATIONS! I found writing my first book the most challenging, and so, if you’re like me, I can tell you that while it’s always hard, it will never be that hard again.
Showing it to your friends and family is an excellent start. If you have writer friends, getting them to look it over and tell you which parts are confusing or dull or too elided will help you revise the book and make it the best book possible. Hopefully, they will also tell you which parts are heartbreaking and brilliant and perfect so that you don’t change those. If you don’t have writer friends, I advise looking into a critique group either in person or online.
Once you’ve revised and polished to your satisfaction, you want to try to find a reputable agent to represent you. You should look up the agents of writers you admire (just google their name + agent usually works) and you should look on AgentQuery.
(Also, as an aside, to be sure any agent [or publisher] you are querying isn’t a scam, look them up on Preditors and Editors, an invaluable resource.)
Once you’ve found your list of agents, you want to craft a query letter. Here is an excellent guide to writing one. A good piece of advice I heard was to write the summary paragraph the way you might write flap copy for a novel.
Once you’ve perfected your query, start sending it out to the agents on your list. Good luck!
And while you’re waiting, it’s time to start your next novel…