One costume that is always popular is the Navy sailor. While there are definitely cultural connotations to the uniform, the Navy also has a long and proud history. Whether you are choosing to pay tribute to the service and history or playing it for laughs (ala Donald Duck) or any other purpose, the uniform itself is relatively simple to construct.
Start with a pair of white bell-bottom pants. There was an unfortunate era in the past that shall remain nameless where these ballooning monstrosities were quite popular, and a trip to the five and dime or Salvation Army will undoubtedly turn up some that were banished from their formerly hip owner's closet after many years of waiting for that nameless era to come back. Have a care not to make them too tight at the waist or thighs as some extreme versions of these pants were. Those aren't sailor's pants, rather a fiendish version of self-torture inflicted on willing victims, but in any case the sailor's white pants should be loose and only gradually widen at the bottom.
Next an A-line white shirt, preferably with blue borders, though these can be sewed on relatively simply, half an inch thick and running along the lapels, collar, and cuffs. A matching blue ribbon should hang down from the V-neck. The lapels should be broad and square flapping over the top of the chest on either side of the neckline. A white dress shirt can be used for this with some modification, but it will not return to its former state, so this transformation would be one way.
Note that the V-neck can be an attractive feature or not so much. If you are extra hairy or have a chest you are not particularly proud of, then raise the neck line up or wear a plain T-shirt underneath. Unless you are going to a swinger's party or strictly for laughs, avoid any hanging jewelry, particularly a giant golden anchor, which seems to be popular among the parody costumers.
Finish off with a white sailor cap ala Gilligan (though avoid any designs on the cap, even the aforementioned anchor). Some prefer a beret, white with a tassel. Finish with some neat white deck shoes.
Note that all the white can be switched out for Navy Blue which is a perfectly acceptable variant, and one that is not quite so prone to attract spilled beverages or other stains. Stick with the white cap, though. Good luck!