Ah, comp titles. How I love to loathe thee.
Seriously though, sometimes choosing the right comp titles is HARD, especially when you're first starting out. There are a lot of resources out there on how to pick good ones, so I'm going to keep things simple here to provide an introduction to the idea for those of you just starting out.
First off, comp titles--also known as comparison titles, help agents and editors know where you think your book will fit into the current market (and where your book would be displayed in a bookstore!)
In general, the guidelines I've heard have been to choose 2-3 titles that were traditionally published within the last 3-5 years, preferably in the same genre. Bonus points if it's an author's debut novel that's done well (without being the book EVERYONE will want to comp). However, even this has exceptions, especially if you're using them for a pitch contest.
Some agents are fine if one of your comps is a movie/show/song title, but not everyone will want that. If agents are on social media, most of them are totally fine if you ask them their preferences for comp titles.
If you're doing a twitter pitch contest, usually putting COMP A x COMP B is totally fine. For the query letter, however, I think getting specific is better. Here's a few suggestions for things you could comp:
-tone
-style
-voice
-pacing
-specific plot points (i.e. shapeshifters, magic tied to artifacts, an underground society, main character personality trait, etc)
That last point is my favorite to use, and has generally gotten the best responses from agents I've queried. Whenever I'm asked, I recommend these types of things to use as comps (and this is especially useful for when you have a book that is unique such as when you wrote your book because you found a gap in the market).
If you're not sure where to start, the library is a great tool. Talk to a librarian! There's also a subreddit for getting book suggestions that can be useful, asking the writing community you're hopefully working on building, using goodreads "similar books" suggestions, and just reading in the genre and age group as much as possible.