First, I recommend that you check with your science teacher to see if there are some specific requirements for the science project exhibit. What are the size dimensions? Material requirements? Three-sided presentation or something else?
Once you have established the parameters, you should begin to look for your materials. Choose a sturdy backboard material. Backboard materials are usually plywood, heavy cardboard, or Masonite. Cut your backboard to the appropriate size dimensions and begin to strategize what material you wish to present on the display. Again, your teacher may have recommendations for this, but students typically include headings for the various steps of the scientific method. Decide whether you want graphs, photos (always a nice touch). Lay down your materials first on the backboard before securing it permanently. Glue works great. You might also wish to include a copy of your paper, so that judges and others can flip through it. In addition, you might want some "samples" from your project, including sand, plants, etc.
For more information and specifics on creating your display, take a look at the two articles referenced below.
L. Compian, Ph.D.
Education.com Expert Panel