You can make a pirate costume by using old clothes and items you have on hand. If you use patterns, use stripes, not flowers, and never use anything that has words, pictures or logos on it. Although pictures of pirates show them wearing long coats with cuffs and tricorn hats, it is not likely you will have these on hand. Vests and head scarves are just as authentic and easier to find.
The basic pirate costume will include a shirt, knickers and a vest. The shirt can be any striped or brightly colored plain shirt available. Make sure it does not have any other print, including words, logos or pictures.
Use a pair of old dress pants for the knickers. Cut them off just below the knee and do not hem them. To make them look more ragged, cut them unevenly and pull some of the threads to create a bit of fringe. If you want them to look more authentic, use pants made with a natural fabric, but don't use jeans.
You can use a vest from an old suit or you can make a simple one. To make the vest, measure across the back of the wearer, from under one arm to under the other, and from the bottom of the neck to the waist. Cut a piece of colorful fabric twice the length of the back measurement and the width of the neck to waist measurement. Fold the two ends of the fabric to the middle. Cut a half circle at the top of each side for the arms and cut each top front corner at an angle for the neck. You can hem all the raw edges, or leave it ragged.
Belt: Use a wide black belt with a large buckle, if you have one. Or you can tie a colorful scarf around your waist. The shirt should not be tucked into the pants.
Head scarf: Fold a large bandana or other square of colorful fabric in half to form a triangle. Put the scarf on the head with the middle point hanging down the back. Tie the other two points in a knot around the middle one.
Jewelry: If you have pierced ears, put a large gold hoop in only one of them. If your ears are not pierced, you make a loop out of a flesh colored thread and hang a gold hoop from it. Hang the loop over your ear.
Eye Patch: Cut a circle of black fabric and poke a hole on each side. Cut a piece of string or a black shoe lace to fit around your head and tie it to the patch.
Check your local used clothing store, thrift shop or Goodwill store. Any of these may have cheap used vests, scarves or shirts that have the look you're wanting for your costume. Dollar stores and hobby stores may well have other useful cheap accessories such as costume jewelry, eye patches, and plastic swords and daggers.