Take photos, lots of photos. From these you'll need to choose your best photos. How do you decide which ones are best? Use the guidelines on the Photography Basics page. Your photos will be graded on the guidelines on that page.
The subject is up to you, but you will need a concept or theme behind your photos. DO NOT just take a bunch of random photos! Plan out what you want to photo. You can do some research on Google by searching for photography gallery shows. Make sure you do this research otherwise you will not understand exactly what you're supposed to do. and your grade will suffer.
You will need an artist's statment to accompany your photos. You should start on this before taking any photos to help you focus on your idea. Use this website for help writing an artists statement. This statement will hang next to your printed photos in the gallery.