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.
Step 2: Making a turret
One red stud (preferably translucent)
6 1x1s with a stud on one side and a hole on the other
one vent piece
one white 2x1 brick
four 2 stud high slants
3 minifigure legs
2 2x1 tiles
2 2x3 tiles
(if you want to make it look like it's shooting) 2 more 1x1 tiles, and 2 minifigure binoculars (these are rare)
2 2x2 plates (white)
To make it look like it's shooting, you take the binoculars and put them on the front of the 2x1s, and take the 1x1s and put them behind. Put the 3x2s on and there you go!
(i now notice that the eye and vent are backwards. Switch 'em around)
Step 3: The Companion Cube
1 2x2 brick
2 2x2 plates
Step 4: Portals
2 2x6 plates (blue and orange)
4 1x4 plates (blue and orange)
2 6x10 plates