Hi _______ ,
Coming up with a list of agents and publishers takes a lot of legwork, but it's worth doing. You’ll pick up a good understanding of how the industry works by going through that process. If it seems like too much, or becomes too much as you go along, let me know and I can find a way to support you.
To get you started, I can recommend some key resources. The Writer’s Market is still the go-to source for writers looking for agents and publishers. There is a Canadian version too (The Canadian Writer’s Market). You can find copies at the public library. Try to get the most up-to-date edition (2019) so you’re not preparing submissions for agents who retired five years ago!
If you work better with online resources, Query Tracker, for a small fee (around $25), gets you access to information about agents in the US, Canada, UK, and Australia. Query Tracker might be missing some data on agents that you could dig up by searching other sources, but its list is pretty full and up-to-date and it has some handy search tools that let you get to the right agents without reading through all the pages of a reference book. It also helps you save and keep track of agents you want to send queries to and the ones you’ve already contacted. This will become important as you send more queries out, but submissions can also be tracked in a simple spreadsheet or on a piece of paper taped up next to your computer.
Agents are individuals and, though the general things they want to see are often similar, each one has preferences when it comes to submissions. Query letters are a given. After that, it varies. Some agents want only the query letter. Some want sample pages (varying from 10 to 50) or sample chapters (1 to 3). Some want an outline. Some want a synopsis. They specify what they want, and The Writer’s Market often has that information, but the best source is usually the agent’s own website.
You could choose to send the same package to everyone. You would get to more agents in less time, and there is a bit of a numbers game to play here. But some agents will get annoyed that you didn’t do your homework and will throw out a submission if it doesn’t meet their criteria, even if it’s a really good book.
What that means is a bit of research and tailoring after you come up with your basic list. The best way to attack that is to set a goal that you can manage, whether that means you send out one query per day, or ten.
I know this is a lot of information to digest. It’s all manageable, though, by taking it one step at a time.
Best of luck!
(By the way, the resources I recommend above have been recommended to me by friends, colleagues, and teachers in the book business. I've used them myself. But I have no commercial relationship or obligation to promote them.)