Reduce is the first word in the mantra. Look for ways to reduce everything. Buy fewer clothes (a color theme will allow you to make more outfits from fewer items), use less water, use less gas powered transportation, in other words: walk, bike or take the bus. Reduce the toxins you bring into your home, the average US woman puts 125 chemicals on her body everyday, most of them are in our beauty products, our skin absorbs over 40% of what we put on it, if you wouldn't eat it don't wear it. I put a website to see what chemicals are in the products you use now. Being organized can help you buy less as well, keep all your school supplies in a central location so you are re-buying things you already have. Reduce packaging, avoid take out containers, and individually wrapped snacks, eat more fresh foods and fewer processed foods it's better for you and the planet.
Reuse is the next step. Rent formal dresses, they are only worn once and you can save a lot of money and probably get a better dress. Rent, swap or download music, movies, games and books. Use the library or websites like paperbackswap.com, netflix, itunes etc.
Recycle is the last step, learn what can be recycled in your area at www.earth911.com. Complete the cycle and buy recycled. Almost all paper products from napkins to binder paper can be made with recycled paper. Plastics can be made into polyester and used for t-shirts and fleece.
Take a look at some of the websites I listed I'm sure you can find more ideas. I also recommend "The Green Book"