When choosing first person you need to remember several things. First is only what the I character sees, hears, smells, touches, tastes can be shown to the reader. The second is not to let your I character go off on tangents. This can muddy the waters unless you want them muddied.
You can choose the main character to be the I character. This can be effective in mysteries especially the cozy kind. New writers often choose to use this view point since writing seems to come easier. For me not so much since I began writing third person, first person became a struggle and I seldom use it. One thing to watch is the use of I. The repetition can be annoying and finding other ways to show the speaker's observations can be useful. Another thing to remember is this character must be humble and not always be praising himself or herself for something they have done. While your reader will identify with the character they may become tired of reading the story. You really can't do much to describe the main character without them seeming vain. So choose this carefully.
Using a supporting character as the first person viewpoint can solve a few of the problems at least with the description of the hero or the heroine This narrator can say things about the main characters without giving the bragging tone to the character. Choosing this viewpoint can keep the reader from becoming bored and he is not as restrained as the hero or heroine in the first person.
First person Minor character which means the narrator is a bit distant from the action. One problem is the reader can only participate in the events where this minor character is present. This brings some restrictions into the story.
First person Shifting. This means more than one character has the first person viewpoint. I've seen this in many New Adult stories. There can be more that two characters using this viewpoint. This may scatter the reader's attention and not be able to identify with any of the characters
So which ever of the variety of first person viewpoints you choose, you need to remember the first two points. What the character experiences and avoiding tangents.
