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Yeahhh since I'm too poor to afford that inflatable one from the ABC shop...

*ahem*... I'd say just get the measurements first, then build the frame out of cheapo wood. (Tools would be useful). Once you have the frame done-... this is where it gets difficult I guess, just cutting and sanding all the outside bits so it resembles the TARDIS. The little windows would be easy enough- just cut holes a bit smaller and glue on small rectangular bits of glass, but the wood on the outside is all different thicknesses and has different edges, maybe if you have a mate or know someone who works with wood.... The doors would be pretty easy, again if you manage to get the outside done. Then just paint it blue and your done. But you could have figured that out for yourself, dunno why the hell I bothered typing it all up...

Actually I think thats a good idea, my uncle works at a joinery (and he's scottish) (not that it was relevant but Scottish is awesome.) Maybe I could convince him to make me one.

Okay, doing set designs for local theater here has caused me to get innovated with some sets. Here are my ideas for you.

First, as others have mentioned, get some basic dimensions. After that, you'll need the following building and paint materials:

lumber - 2x2 pieces, probably six to eight, 10 ft lengths being the best as you can cut them down to size (try to get them as straight as possible).

1/2" builder's stryofoam (here in the States, it's pink in color and has a very thin plastic covering which you don't remove as you will need to paint on this side) - these come in 4'x8' sheets and you'll have figure how many sheets you'll need based on the dimensions. This is very lightweight, fairly durable, easy to cut into shape (use a sharp straight knife and "saw" through it; when gently sand it with fine grit sandpaper... wear a mask when you sand).

white glue (Elmer's is good; the yellow wood glue is also okay but the other can be obtained in large portions for cheaper).

wood screws (I don't use nails to build any of my structures; small box of 50 should be)

hinges (for the door; you'll have to figure out some other hardware ideas, like a latch or the like, if you want the door to stay closed)

paint - you'll need a gallon of Kilz (or similar product; get the water-based product, not oil!) to prep the stryofoam for painting and probably a gallon (not oil-based) of the TARDIS blue match (a paint store can match the color for you if you take them a color sample). You'll also need much smaller amounts of various colors for detailing the door, the windows, the sign, etc. You'll want either flat or matte finish paint.

Now building...
(1) Assemble each of the sides and the door (simple rectangular frames of the 2x2s which have been screwed together). Also do the same for the roof.
(2) Place the stryofoam over each of the pieces and cut them to the size of each frame. Set them beside each frame. Apply lines of glue to the frame and then put the stryofoam back on, making sure that the plastic side of the foam is up. Wipe any excess glue from edges. You'll need to put a little weight on the attachment edges (some books are good). Give the glue at least 24 hours to harden. Gently sand the edges of the foam after it has hardened to the wooden frame (wear a simple face mask while doing this).
(3) Assemble the frames to create the TARDIS. You can use the wood screws through the foam. Just go slowly. You'll also screw together the three side first, then the roof, and lastly the hinged door. You'll need help with this process.
(4) Prep the stryofoam for painting with the Kilz. Do this in a well ventilated area as the "odorless" product really isn't. Probably one thick coat will do; you want to have little of the pink showing. Allow this to thoroughly dry.
(5) Now you can paint your TARDIS blue. Probably two even coats, allowing each application to thoroughly dry. Then do your detail work and any touch-ups.

This is probably more info than you needed. But anything to help a fellow prankster! biggrin

There used to be a tech manual that had the TARDIS dimensions in it. I do not know if it is still in print or not, but you can look around for it.