If you want more than just Chell (the main character), a Portal Gun, and portals, then here is your help! I have come up with a turret, Chell, a companion cube and two portals, not to mention a test chamber, so let's get started!
one of the things you'll need is a hot glue gun. You won't be able to make most of the things if you aren't willing to glue your legos to the objects.
Step 1: Making Chell
1 brown (or otherwise) girl hair
smiling head (or otherwise)
3 gray studs (people call them dots for some reason) and 1 translucent stud (blue or orange would be good)
One circular 1x1
3 antennae (you can get them from lever pieces with the gray bottoms)
and 2 1x1 Orange translucent (or regular) tiles.