As a teenage girl, what steps can I start taking to avoid aging skin and wrinkles in the future?

Don't smoke and... The most important thing is DON'T SMOKE tobacco. Avoid excess sun exposure (esp. avoid sunburn). You are what you eat so for healthy skin make sure you eat a healthy diet rich in fruit and veggies and avoid excess sweets/simple carbs and fried foods. Exercise. Do your best to minimize stress. Meditation and/or yoga or chi gong can help. Also take a quality multivit. with extra D in fall & winter.
Protect from sun. Wrinkles are almost exclusively the result of sun exposure accumulated over years, not age. Smoking also causes skin lines to form. Look at most elderly nuns, how smooth and pretty their skin is. They have almost never have spent time out doors. Apply a good sunscreen every day, be healthy, and your skin decades from now will look tremendous. Dont waste money on wrinkle products-they DONT WORK.