For those of you unfamiliar with LTUE, it's Life The Universe and Everything, an annual science fiction and fantasy symposium (although I see no difference between this and a conference) held in Provo Utah. This year it was actually in American Fork. It's mainly for writers, but they have illustrators classes most years and a teacher's symposium is linked to it. This year they had a gamers section as well.
On the first day of LTUE I attended a class on first person POV. So you get my notes. :)
In first person POV you can generally have only one MC (I'll cover the alternative another time). Everything needs to be filtered through that one person's viewpoint. If you want to hide things from your readers it's relatively simple--just don't let your MC know. Because if your MC knows, so will your readers. In essence, 1st person is a 100,000 word monologue.
Of course, that makes it relatively simple to trick your readers, if that's your goal. If your MC is an unreliable narrator, believes incorrect information, or simply is clueless, your readers (who see everything through the filter of this character's thoughts) will also be gullible, incorrect, or clueless. But if you use your 1st person MC as an unreliable narrator, make sure it's deliberate! Just like in any other form of writing, these decisions have to have a purpose behind them.
Another possible problem with 1st person is the introduction of the narrator. In this case, your MC is your narrator. Therefore, anything that comes through another person's perceptions is anathema. Everything has to be from the understanding of the MC. If your MC smiles, that's one thing--that's something he or she can know. If h/er dark eyes narrow and flash angrily--sorry, MC would not be able to see dark eyes flash unless looking at h/erself in a mirror. The MC cannot see the actions of people behind h/er, or in another room, or determine their motives except through h/er own perceptions and understandings.
A narrator might see dark curly hair lying along her shoulder, or smugly cross his ankles in their new name-brand running shoes (if this character would be proud of such a thing). You need to introduce your MC through h/er thoughts. Does the narrator see some special significance in being in an AP English class? Is s/he a jock and see's other people who don't play sports as inferior? Is s/he a lawyer and walks down the street wondering who's suing who for what? The checklist (I have blond hair and green eyes. I am 5' ll" and I play basketball) should be avoided at all costs.
In order to do this, you need to know your narrator inside and out. Consistency is absolutely necessary, even if your narrator is unreliable. If you're a pantser, make a list of your character traits as you go so that you can reintegrate those things after the fact. If you're an outliner, consider making a list of character traits and mannerisms before you start. You also need to know your character's age, speaking styles, and history, which leads us to the next point.
Make sure that your MC is a distinct and interesting individual. Having the hopeless nerd as your MC might be boring, but if your hopeless nerd is a budding psychopath that adds a level to it (and incidentally one which can only be experienced through 1st person). The bag lady on the corner might not be interesting in third person, but then we see her thoughts and realize that she's a spy using this as a disguise. Or thinks she is.
Using first person can create a sense of immediacy that is difficult with third person POV. Your readers have direct access to this character's thoughts and feelings, and a stake in the MC's continued survival--the brain is reading "I" and equates this with the reader directly.
Which is a topic for another day, because this is too long and I've covered barely a handful of the points in my notes.