Try to get surfer girl hair. It is not true that all surfers have blonde hair because it is bleached from the sun. Brunettes don't go blonde just because they spend time in the sun. However, surfers do have lighter hair than others because they spend a lot of time in the sun and saltwater makes your hair lighter - think Fabiola Gatti.
A good way to achieve this look is to mix one part of sea salt with three parts of water, and spritz on your hair using an old hairspray bottle. It will give you that care-free "just surfed" look and is a good, natural alternative to dye if you can't actually go surfing all the time. If your hair is naturally wavy or curly, embrace it - surfer hair is typically tousled.
If it's naturally straight, after towel drying hair curl it in rags/french plait it and leave it overnight and it will be wavy by morning. There are some products on the market that boast 'surfer hair' so try these out and see what they do for. And also, if you have darker hair, or hair that's a little more thick, then you should try putting it in a braid, or braids. Or put you hair in a bun, or a side pony tail, or just leave it down.
Get a tan. It's essential for that laid-back, carefree look and looks flattering on almost everyone as it is an automatic slimmer and makes you look healthier and radiant. Real surfers have real beach tans, but of course if you live in a climate where you need to wear a wetsuit, it's very easy to get that unflattering 'wetsuit tan' where past the neck you're completely white!
Additionally you might live in a cooler climate not near the beach, or it might be winter, or you may sunburn instead of tan. Here, the only solution is sunless tanning, either through a sunbed (not recommended), or faking it. Make sure you use a good quality tanner, exfoliating beforehand and moisturizing afterwards to prolong it. And if you have darker skin, then you don't have to tan, unless you still want to. But your skins already pretty.
Sort out your wardrobe. Your clothes may include surf brands like Billabong and Roxy, but you shouldn't wear only surfing brands. Real surfers don't always live in beach brands, it can be too expensive and they're not always practical.
The usual outfit for a surfer girl is boardshorts and a bikini, but for school or during cool weather, surf-inspired tops and jeans are a good look. Also, many surfer girls are bohemian in style and opt for the 'boho chic' look (see Sienna Miller, Mary-Kate Olsen for ideas), which means gypsy inspired clothes, long skirts, maxi dresses, floral prints, lots of bangles and jewellery, and layered clothing.
Girls in Hawaii usually wear lots of beach coverup dresses. What lots of girls wear are jean shorts with any tank, tee, or hoodie, with a neutral colored leather belt, trucker hat, and Vans or converse slip ons, or flip flops.
You should always have:
plenty of denim shorts
a few pairs of jeans
lots of basic tank tops and tees
a few sundresses in floral prints or bright colors
2 pairs of cargo pants
Wear jewelry such as bracelets made of hemp, rope, string and seashells, and necklaces made of the same materials or rock pendants.
Paint your toenails a bright color so your feet look nice - remember they'll be bare on the beach. Most surfers paint their nails. Good colors are bright colors! Teal, aqua, orange, red, hot pink, and sunshine yellow. But remember, sand and nail varnish don't mix. It gets scraped off really quickly.
Live the life
Make sure to be yourself at ALL times and be the happiest person in the world.
Source: Wikipedia, Girls On Boards, Wikihow.com