There are many resources for finding agents and narrowing down who should make your query list.
Where to Find Agents
Look at books that are similar books to yours and go to the acknowledgement section and see who the agent is. (Disclaimer- agents won't pick your book if it's super close to an author's book they already represent)
Use websites like QueryTracker, Writer's Digest, Manuscript WishList, Publishers Marketplace, Literary Rambles, WritersServices, Twitter.
Writing Conferences.
Look at pitch contests on Twitter.
Search #MSWL and #AskAgent on Twitter. #TenQueries or #10Queries is a great tag for learning agents' thoughts on queries.
On MSWL you can search for key terms for a faster search on what agents are looking for like 'retelling'.
On QueryTracker, if you pay for the service, it'll tell you other agents like the one your looking at - great for building your list.
Choosing Who to Query & Keeping Track
Using Google Docs or an Excel spreadsheet to note the agent name, agency, who they represent, and what genre and age they represent is helpful in keeping track. This is also shown in Query Tracker.
Make sure the agents are currently selling what your book is. Some may be asking for YA fantasy, but if all they've published lately is adult romance, they may not be your best bet.
Query Tracker lets you see what that agent is requesting. Publishers Marketplace lets you look at what they've sold.
How I Sorted Agents
My top agents (A list) are ones that represent multi-genres and age groups because I don't want to stay in one lane. I'd also love an editorial type agent.
I'm also looking at their agency, how new or established they are, if they have a great senior staff to help them, any special MSWL requests, etc.
Also note which agencies are No For One means No For All (N1NA), which means you can only send to ONE agent at that agency. They say they pass it off if they think it'll fit another agent. And a few agencies have it to where you can send to the whole agency, and they'll pass it off to the agent that's the best fit.
You can also sort agents by response time: fast, medium, slow. Sometimes you want fast responders up front to get a feel for if your material is ready or needs tweaks. Choose fast responders on your B or C list, and keep your A list for when you're super sure your materials are getting responses.
I list my agents in an excel sheet with columns for agency, agent name, twitter handle, website, QT form vs email, sub materials, genres, ages, any MSWL I could match on, any other notes, response time, my A, B, C list, my query date, and response. I used to make a list in the order I would sub to, but now I go by agency name and color code by query batch, keeping in mind my A, B, C list.
Querying Tip
For your first round, send out 3-4 test queries to fast responders on your B and C list. If you don't receive any requests, look at your query package. If you get 3-4 requests, you're likely good to send to your A listers. If you get 1-2 requests, continue sending to your B or C list, and look at your materials to see what you can tweak. Some quieter books might get a lower request rate, but that might be okay for that type of book.
When you get a lot of full requests, start sending it to your A list, especially if an agent requests a call. Send those queries out so that when that agent offers, then you can email all the agents who have your query/pages that you have an offer. Never email your query with an offer of representation.
Good luck!
Message me if you have any questions or comments!