Here are instructions for two easy to make homemade trumpets. The first can be made with stuff found around the house. The second will require a trip to the hardware store and the music store. At the end of the step-by-step plans you will find basic instructions on how to play your homemade instrument.
You can find a trumpet mouthpiece on eBay that will work great. The 4' of plastic tubing and metal or plastic funnel can be found at a do-it-yourself hardware store.