1. Do I have a hook?
Also known as the elevator pitch. Can you summarize your story into one sentence? Is it an interesting sentence? Is it so interesting that it would stand out among a pile of thousands of other queries? Your hook needs to clearly state the conflict and the stakes. A man is about to commit suicide when an angel shows him what his town would be like if he had never lived. What's the conflict? A man is so tortured (doesn't matter by what) that he wants to end his life. What are the stakes? A man's life and apparently the future of an entire town. Who are the characters? The man, the angel, and on the periphery, the entire town (they'd have to be otherwise who would care about what happened to them, right?).
2. Do I have a platform?
It used to be that you would have to publish some short stories in respectable magazines or journals before you had a real shot at an agent. Nobody offers that advice anymore--because most of those magazines and journals are now defunct and even if they weren't, the book buying public sure doesn't read them. These days you need to have an online presence, you need to be able to reach thousands of people, and you need to prove that you're not going to fade into the night. Taking the time to keep up a blog and a decent webpage is a wise investment.
3. Is my manuscript actually publishable?
There are many, many reasons the acceptance rate is so low. The biggest one is that there are a finite number of slots and an infinite number of submissions. But most of those submissions aren't publishable even with endless resources. Stephen King didn't collect hundreds of rejections because editors are stupid. He collected them because he wasn't a great writer at first. You need to step up your game as well. Don't worry about making it perfect, just make it good.
4. Is my manuscript actually publishable?
I'm repeating myself, but it's a different question. I promise. See, agents might reject your book due to the writing style, or they might reject it due to the content. Two years ago, a story about a teenager and her vampire lover would have been highly desired, but now that the market is a bit flooded, editors are looking for the next big thing. Which means agents are. Which means your story about a teenager and her vampire lover has to be really, really damned good to get noticed. Write the story you want to tell, but don't be ignorant of the market.
5. Do I need an agent?
You may not. And the thing of it is? Some of them are starting to recognize that fact. I'm not going to say agents are completely unnecessary, because I don't believe that's true. They serve a very valuable function for authors and for publishers, and some of my favorite people happen to be literary agents. But I'm no longer convinced they're necessary for everybody. Mainly because for the past five years, I've been making a perfectly respectable living as an author and I've never been able to snag an agent.
Who is your market? What publishers do you want to submit to? What are your long term career goals? How will an agent help you reach your goals? Be sure you understand what you're looking for and why before you start querying. Don't just look for an agent because it seems like the thing to do.