CONGRAULATIONS on reaching this (very exciting) stage. Well, it seems you are already are ahead of the game in that you have taken the time to do your research (which sadly writers often don’t do). That has already increased your chances by, like, a million. 🙂
You will probably already know these tips, but
1) thoroughly research the agents you want to submit to and try to tailor your cover letter to them as much as you can. Any small mention of why specifically you are submitting to them will go a long way
2) follow their individual submission guidelines to the letter (obvs)
3) Suggested content / format for a cover letter:
Dear [Agent X],
I am pleased to send you the first three chapters and synopsis of [TITLE], a [word count] [genre] novel about [elevator pitch]. I though this book might appeal to you as you mentioned in [tweet / blog / profile] that you like [area of interest that your book ticks]. More generally, I think the book would appeal to readers of [comp titles / authors].
[1-2 paragraph BLURB]
[BIT ABOUT ME: general facts; anything that gives you ‘expertise’ in writing the novel (e.g. it’s set in Cumbria; I live in Cumbria); any common ground with the agent; any awards / prizes / previous pubs. Mention if you have any full MS requests from other agents]
Thank-you for your time in considering my work….
4) This is a great video for how to write a pitch blurb (okay, I’m biased, but still): https://www.youtube.com/watch?v=eMie0mX4QfM
5) Don’t name other agents in your cover letter (e.g. don’t say “the full MS is currently with X”). I did that, and only now do I know it was a faux pas! (It all worked out okay though)
6) Be nervous, but still send it out. Make a shortlist of up to 12 agents, and send out an initial batch of 5-6. You can then tweak your sub based on any feedback before sending out again to new agents.
Phewf! There’s probably lots more I could say, but don’t want to ramble on. Happy to answer any questions / look at your cover letter or synopsis, whatever help I can be