A pirate costume for a female has more opportunities for variation and customization than a pirate costume for a man, and considering that a man can choose from a Captain Hook-like villain complete with tailcoat and hook to a anti-heroic knave like Captain Jack Sparrow to a swabbie with a head scarf, a scope and a cutlass--there is a lot to choose from. Certain basic ideas permeate all the costumes such as a tan and some sort of knife or sword, but there are enough looks out there to accommodate most tastes, levels of modesty and budgets.
There are a lot of different looks to go for when picking a top for a girl's or woman's pirate costume. For a simple and traditional pirate top, choose a loose-fitting button down shirt with wide sleeves, the more weather-looking the better. Neutral colored blouses with a coarser material such as muslin work just as well. Add a vest to define the waist and bust as desired. Alternatively, choose a horizontally striped shirt. For a more captain-like look, find a slim-fitting, thigh length jacket in a darker color at a thrift store and modify it with metal buttons and metallic trim at the sleeves.
Females can wear either pants or a skirt for their pirate costume. If you want to wear pants, wear loose fitting pants either cut or bound under the knee or tuck tighter pants into boots. For a skirt, wear a circle skirt. For added effect, wear two skirts one on top of the other, the darker one on top. Pin the darker skirt up in places and dip the hem of the lighter shirt into black tea and let it dry to make it seem dirty.
If you are wearing a coat, a tricorner hat is a must. Otherwise, a simple headscarf will do. A belt with tools of the trade, at least a cutlass or knife, but also a spyglass and heavily worn treasure map too, if possible, helps create the image. Use bronzer and an artificial tanning lotion to create the "at sea" look and use brown eyeliner to create simulated dirt of the arms and cheeks. For an adult costume, you can go for dramatic dark liner for a more exotic look. Add beads to the hair to continue the trend.