1) On makeup:
You can get a lot out of Youtube tutorials, but there's nothing like an in-person consultation.
You might make an appointment with a makeup artist for some techniques and training. The artists at some of the makeup counters in department stores will often be familiar with makeup techniques for costuming and theatre - for instance, the MAC counter in a Macy's or Dillard's will almost always have a couple of very skilled makeup artists who will be happy to give you some tips.
Bear in mind that it's part of their job to sell products. The mid-tier and higher-end products like these will be somewhat more expensive than the off-the-peg stuff from Walgreens, but will cover better, look better, and last longer. However, if you only use it on rare occasions (e.g. costuming), I don't know about the shelf life.
Why don't you swing by a MAC counter, ask around, see if any of their artists are versed in costume and stage makeup, and would be willing to chat with you about what you need and how to use it? .. Make sure you tell them you're specifically looking for products and techniques suitable for costuming, that you probably want some basic/universal things that will work well with your skin, and maybe a couple of other items specific to the costume you're pursuing. Bring reference images. ...and if you're on a tight budget for products, let them know that up front.
2) On posing.
The "comfort" in posing just comes with practice and experience, but there are some things you can do to improve in the meantime.
Before the photoshoot, get yourself into the mindset of the character. Be the character, not yourself.
Practicing in front of a mirror definitely helps. Better yet, practice with a friend holding the camera and taking pictures - even if it's just a camera phone or point-n-shoot, you can get valuable experience in seeing what you look like in a photo. (it's not always the same as what's in the mirror)
An experienced photographer can help a lot too, with posing and facial expressions. As a photographer, I find it helpful to show off the pose that I want from the subject - this is almost always good for a few laughs, especially when I'm trying to demonstrate sexy or feminine poses. When searching for a photographer, ask if they're experienced in giving posing direction.
Tell jokes, or find some other icebreakers. It's not much different than any other social situation, really... except in a photo shoot, both you and the photographer want to make the best of it. Try not to feel self-conscious; nobody's judging you.
If all else fails, "fake it till you make it."