Choose suitable footwear. Ideally, you'll need some tall black boots. They cannot be heeled and are best with a rubber tread along the bottom, to make the boots look more athletic than stylish. Also, they should not have any embellishments other than a simple zipper up the side of the boot.
Arrange your hair suitably. If you already have brunette or black hair, you won't need a wig. Braid your hair in a diagonal Dutch Braid across the back of your head if you want an ideal Katniss look from the movie, but if you want to go for an easier braid, just do a regular braid down your back (which is from the book). The Dutch Braid from the movie is most recognized, but either will give you a good Katniss look. Secure with a ponytail holder.
Find a copy of Katniss' signature Mockingjay pin. These are sold online, such as through online auction sellers. However, you could also make one by finding the design online, printing it off and attaching it to strong card. Use varnish suitable for paper to protect it from light rain (just in case) and hot glue a brooch pin to the back.
Find a mint or light green or black collared blouse or polo shirt. It should be worn tucked into your pants. You can get a shirt like this from an online staff uniform supplier or from a thrift store.