This is the #1 question I get asked almost every day. It took me a really long time to examine my past and fully understand just how I did it. Honestly, I never really "tried" to get into beauty - I didn't even realize how interested I was in makeup until people started asking me about it. I always assumed it was normal for girls my age to spend all their spare time and energy researching and testing out makeup.
Of course, I did already complete the "My Makeup Story" tag on YouTube, which can be viewed below...but that's an 11-minute video. As a lover of brevity, I figured it would be more than courteous to also let you read the story.
Basically, I got my start with makeup as an art form by reading magazines geared toward girls my age. I wasn't allowed to read "big-girl" magazines like Teen Vogue when I was little, so I'd read other magazines and (literally) memorize the articles about hair and makeup. I could still tell you how Girl's Life told me to achieve a vampire-perfect look, how to master the itty-bitty Katy Perry eyeliner wings, and how to style my hair on a lazy Monday.
I also practiced my hair and makeup techniques on anyone who would let me. I practiced on myself, especially before bed and before showers. (Please learn from my mistakes: do not test anything dangerous before bed. It turns out that waterproof liquid eyeliner doesn't work too well on my waterline, a bad situation that turned ugly when I was digging clumps of liner from my eyes at 2 AM. It wasn't fun.)
I'd definitely say that any aspiring makeup artist (or any girl who is just looking to expand her skills) should start by learning and testing. Aside from magazines and articles on the Internet, there are some awesome books out there about makeup.
For the super-beginner, I'd recommend Crazy Busy Beautiful by Carmindy. It covers all the basics of beauty and will give you a solid base of information to simplify your routine. This book also does a very good job of simplifying terminology and teaching newbies how to move out of their cosmetic comfort zone. It's entirely illustrated, which means no photo demonstrations, so I wouldn't recommend this to anyone who currently doesn't wear any makeup, but it's great for people who already have a little bit of knowledge and are looking to learn the basics. It's the size of a novel so it's great to just throw in your purse and read a few paragraphs whenever you have a spare moment.
If you already have a pretty good handle on makeup, you probably have a harder time finding sources of new information. Makeup Manual by Bobbi Brown is an amazing book. I've read this book cover-to-cover several times and I have the pages mostly memorized. There are a lot of basics as well, so I'd recommend this to the beginners and intermediates alike. The book itself is a little large so it's definitely not something you can run around town with, but it's great for a bedtime read, or just to flip through when you get bored.
When you get started with beauty, always keep in mind that you're beautiful without makeup. Cosmetics are just a tool to help boost your confidence, and your "naked" face is just a blank canvas - NOT a "before" picture. :)