First person point of view (POV) is when the narrator uses first person pronouns (I, my, me, etc.).
As I was putting the milk onto the conveyor belt, the blonde cashier smiled at me. She had the cutest dimples, and I got so flustered that I dropped the milk. Of course the milk hit a metal corner on its way to the floor and burst everywhere, including all over me. I wanted to melt into the puddle and never come out.
A first person narrator is most often the main character of the book, but it doesn't have to be. It could be a secondary character or someone outside the story, as long as they are using first person pronouns to refer to themselves.
First person POV tends to be more common in YA books than in the other age groups. It lends itself well to toward inner dialogue and YA books are often about big life decisions and discovering who you are. It also is relatable for teenagers.
After YA novels, romance novels probably have the next highest rate of first person because it works well to keep readers close to the main character and show their perspective on the love interest. But first person can be used in any genre as long as it is done well.
I recommend first person POV for stories with strong main characters, particularly sassy or funny ones (like the Percy Jackson series), or for people writing coming-of-age stories. First person also works really well if you are interested in using an unreliable narrator.
Make sure the narration is still clear and readable without losing the narrator's voice. You can do this by imitating, but not replicating a realistic style of talking. A sentence can sound completely different written rather than spoken due to factors such as tone of voice and facial expressions. You may have noticed this when trying to communicate over email or text messages. So your narrator's voice should feel authentic without actually being a transcription of how people talk. If you've ever done any transcription, you'll know why that would be the absolutely worst to read. People pause in weird places, make small grammatical mistakes, repeat themselves, and do dozen of other things when they talk that would make your writing genuinely painful for readers, as well as confusing.
To write first person well, you need to be careful about not letting the narrator come across as whiny. In first person books, it can be hard to show a character going through something deeply emotional without getting stuck in never-ending, woe-is-me inner monologues. In The Hunger Games, first person works well because Katniss specifically tries to ignore the tragedies around her and focus on her own survival. Otherwise her grief and the horror of the things she faces would risk slowing the story down.
I don’t recommend first person POV for books with large casts of characters like Lord of the Rings. First person books can have multiple POV characters, but it can be harder for readers to follow multiple characters because who "I" is keeps changing. If you choose to have multiple first person narrators, you will likely want to limit you POV characters (I recommend no more than two), label your chapters with the name of the current POV character, and avoid switching POVs midchapter.
One tricky thing about your main character narrating the story in first person can be the use of tenses. Is the character telling the events in real-time as they happen? Is the character retelling the story having lived through the ending? This can trip some authors up.
You can have the narrator telling events in real-time in both present and past tense. To do this, make sure your narrator has no facts that they didn't know by this point in the story and reacts to things in the moment, without any perspective or distance from the events. Some writers worry that using past tense means the character has to know the end already, but readers don't really think of that unless you specifically introduce the idea to them. (For example, the beginning of The Red Pyramid by Rick Riordan sets up the characters to be narrating from a specific time and place after the events have concluded.)
Another concern that I see a lot, is that readers sometimes really hate first person present tense. There are definitely readers who feel that way, but The Hunger Games was a bestseller, so clearly not all readers feel that way. Instead of trying to please all possible readers, I recommend choosing your POV based on what feels right for your book and feels natural when you're writing.