you need to get a plan/pattern
try looking online for dimensions etc and use a scalpel to cut out the Styrofoam.
References :

How to Build a Glider

By eHow Hobbies, Games & Toys Editor

Rate: (31 Ratings)

Gliders are easy-to-build, small aircrafts that require only a simple hand toss to get airborne. You can build these aircrafts out of materials ranging from Styrofoam to wood. Here are some basic instructions for building a simple foam or balsa glider aircraft.

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Instructions
Difficulty: Moderately Easy
A Styrofoam Glider
Step1
Visit NASA's website for a template of a glider you can make out of a Styrofoam tray. Print and cut out the pieces for the pattern, and tape them onto the tray. Trace the pieces with a knife to cut them out of the foam.
Step2
Smooth any rough edges with an emery board. Cut slots into the middle of the main body and the tail section of the glider aircraft. Slide the wing and stabilizer pieces through the slots, fitting them evenly through the holes.
Step3
Toss the glider to see how it flies. If it doesn't fly well, add paper clips to the front or the nose of the aircraft for added weight.
A Wood Glider
Step1
Go to the website of the Society of Women Engineers to get two balsa wood patterns for a glider. Choose from a straight wing glider or a sweptback wing version.
Step2
Cut the pattern and place the airplane pieces on a sheet of balsa wood. Trace around the cut outs with a craft knife. Since the patterns here are designed for several types of materials, make the slots for the wing piece and the stabilizer a little smaller than they're shown on the pattern.
Step3
Sand the rough edges of all the cut pieces. Add a small drop of wood glue to the tail section and the middle of the fuselage to keep the wing and the stabilizer in place. Ensure they're even in their slots before you glue them.
Step4
Perform a test flight of your glider aircraft. If it doesn't fly well, add some weight to the nose of your glider. You can use paper clips, or you can glue a penny or some other small object to it.
References :
http://www.ehow.com/how_2121468_build-glider.html?ref=fuel&utm_source=yahoo&utm_medium=ssp&utm_campaign=yssp_art

it's really an ambiguous question. How big a piece do you have? do you want it to fly or just look nice? what sort of Styrofoam is it?

If you don't have a kit and proper direction, to get a flying model is nearly impossible unless you have a pretty basic understandings of how a plane fly's.
Cut out your wings, fuselage (with rudder) and stabilizers with a ratio that looks right.
Your going to need to put on a little bit of weight on the front of the aircraft, and place the wings on the fuselage so that the front and back are balanced when you hold the plane up on the tip of the wings.
References :

Powered, glider? Size limit? Weight limit? Biplane, monoplane? High or low wing? Dihedral? Adjustable rudder, elevator, ailerons? Read up on aerodynamics, CG, center of lift, Reynolds number, wing aspect ratio.
Cut foam with hot wire. Lots of webpages on that. Sculpt surfaces with cheese-grater style body repair tool. If you know someone making surfboards, they have this down already.
References :
http://usrainfo.org/rankin_article2.htm
http://www.4ppl.com/blog/entry/_Aerodynamics_How_To_Design_A_Wing_To_Be_Just_Like_NATIONALflt0477_Lol_
http://www.modelbouwforum.nl/forums/algemeen-modelvliegen/46394-vleugel-oppervlakte.html
http://www.paperplane.org/Aerodynamics/paero.htm
ground school, flying in an RF-4B