For a simple, put together look, try this hairstyle that Hanna sported in season 1. (It would probably work better for girls with long straight/wavy hair, but curly haired girls, don't fret, you can easily do it too!)
Just take 3 strands of hair, and starting at the crown of your head, french braid them into a headband. Continue with the braid until you get about 2 inches to your ear. Tie the braid so it doesn't fall out, and use bobby pins to pin the braid underneath your hair, to make it seem like the braid never ends. Perfect for school, a party, the beach, poolside, etc.
For girls with longer, wavy, curly hair, this look, worn by Aria, is much easier to create, and can be maintained throughout the day. Start with your hair down. Then, take about an inch of hair right from the front, on both sides, start braiding them, however you want, fishtail, regular, french, etc. Once that's finished, bring the braids around your head, so that they are in the back. Use a small baby clip, and voila, a simple, yet cute look. Perfect to keep long hair out of your face on days when you don't want to wear it up.
For Spencer's simple wavy look, seen below, tousle hair with fingers by flipping it over, using some hair spray. Don't use the sticky kind, for this look, you want to use the "beach waves" kind, such as "Not Your Mother's Beach Babe Texturing Sea Salt Spray," which can be purchased here.
Add some hairspray when your head is in the upright position for a more stable hold.
The newest craze in hair styles is the ombre, which Emily sports a lot on "Pretty Little Liars." The one thing I love about her character is her simple hairstyles, which can be done in less than 5 minutes. In order to get the ombre effect, all you need to do is buy any hair dye from your local drug store, pharmacy, etc. I would advise using something a little lighter than your natural hair color in case it doesn't work right the first time (so it's not that noticeable). The best one to use is Clairol. Once you figure out the ombre effect (for tips, visit http://www.wikihow.com/Ombre-Hair), use the hair spray from above to get the bouncy beach waves!