I didn't start using makeup til I was around that age, really. So don't feel too embarrassed.
There are a few things you should know. Cheap makeup is usually
a waste of money, ESPECIALLY if you're looking for SFX makeup. You should budget for it the same as you'd budget for nice material, wigs, or anything else. However, don't think of it as draining out your life savings--it can be an investment that lasts a good while and honestly it's a lot of fun when you're bored.
Bear in mind the following is all stuff I discovered on my own--I'm no makeup artist but I've been friends with some. Those are my qualifications heh.
First things you want to pick up are the basics: a good foundation, powder (some foundations come in powder so you can hit two birds with one stone--I personally LOATHE liquid foundations) eyeliner, mascara, lipstick. Branch out from there to the complicated stuff like eyeshadows, blush, concealer sticks, false eyelashes, etc.
My personal favorite "cheap" brand for foundation and powder is Physician's Formula. I don't break out using this and the organic formulas are very light and good for everyday wear--for me. Remember skin is different for every person! So while I might say "This stuff is awesome!" someone else might absolutely hate it.
Plus I am super ghost pale and they have a shade that works great on me. I love their little powder sticks with the brush attached to it. I think I paid like $13 for mine, so this may be the most expensive part if you're going the cheap route.
For eyeliner liquids look great but are such a bitch to learn to use. Definitely get some remover because scrubbing it off can be painful! I really like the twist up sticks they're making now--you can get similar results--not identical-and you don't have to worry about sharpening. They can run as little as $6.
Mascara tends to be more expensive. Go for a non clumping kind. I've had luck with... it's in a green tube. ;_; Also ran about $7. Don't buy a cheap brand here--they can get so freaking messy.
For lipsticks I found you CAN go cheap. Sally's Beauty Supply carries tiny samples for a buck and they tend to look good, though if you've ever worn--even as a sample--something like Mac's YOU KNOW THE DIFFERENCE. lol. Be careful picking out colors. Just because you think dark purple is freaking awesome, does not mean it will look good on you. Just because it looks like it is a red, does not mean it will show up red on your lips--especially true of the cheaper ones.
Um... Other stuff. Play around in an Ulta's or Sephora's if you have one near you. There's some interesting stuff you may not realize you're interested in. Also some department stores will give free makeovers and tips without doing more than ENCOURAGING you to buy things.
As for costume makeup... Hehehe... that's a whole nother ballgame, so much to cover... hopefully my ramblings help a little?