Congrats on deciding to querying an agent. My advice?
a) Ask a published author for advice.
Publishing is a serious business and there's not always a guarantee that everyone will give you an answer in your best interests. Some might even be misleading. I would suggest asking Zoe Marriot at zdmarriott@gmail.com. Remember to put [Alexandra] in the subject name. I suggest this author to you because she is an excellent blogger and a fantastic writer. She's based in the UK and even though she isn't well known, she is very helpful when it comes to answer general questions about publishing and writing.
If not that, then google for advice by well known authors - present day. Veronica Roth, Holly Black, Richelle Mead, James Dashner, Stephenie Meyer. There are hundreds of options. Almost all writers blogs nowadays.
ALSO, take a look at the Writer's Digest website. It is a fountain of knowledge, all by experienced, published writers.
b) You're not 'up against anyone'.
It can seem competitive at times but trust me, you're not up against anyone. I understand your doubts but the thing is that most authors who are published in their late or early twenties start sending out queries at this point. This is your stepping stone. Chances are you will be rejected the first couple of times. Don't get disheartened. If the rejection offers you advice, TAKE IT. These people know what they're talking about and you NEED their advice.
c) Guide to Literary Agents.
Google it and look at it. Make sure, again and again, that you are sending your manuscript to the right agent. Also, I'd suggest that you pinpoint lesser known or new agents. Why? Because in a way, they are also looking for writers to represent. They need GOOD writers and if you've heard good things about any relatively new agent, send a query now! Always be aware of scammers, of course.
d) Don't be intimidated.
Again, it is understandable but you need courage to firstly finish a book. You did that! Obviously, getting an agent can also be a little terrifying because that's when the publishing process seems more real and you realize that if you get a book deal, you lose control over your manuscript and its on its way to people's bookshelves where they will interpret however they like. They'll love it or they'll hate it.
Just shove aside your doubts and query an agent. If you never start, you're likely to be stuck for ages.
View agents as something endlessly beneficial for you. Always remind yourself that Rowling tells all writers to get agents. Why? Because they're good for you. By not getting one, you're reducing your chances of getting published and that's unfair to you.
Remember, Veronica Roth is only in her early twenties but she got published and made her way onto the NYT bestseller list. Give it a shot. You won't regret it =)