Elves come in many different contexts. So, there are a variety of elf costumes you could chose.
Most elf costumes start with a green tunic over leggings or tights, and a pointed hat. Optional elements include bells (especially for Santa's elves), boots, a bow and arrows. Santa's elves may wear red, but most elves wear clothing in forest colors -- greens and browns.
A rough tunic is easy to make with a large rectangle of cloth -- put a hole for your head in the middle. The long ends hang down in front and back. Use a belt or another strip of the cloth to tie down the tunic and/or sew the sides together.