Hmm, first prop. Man, I know I made many toy weapons (space swords, laser rifles, and ghost busting equipment) back when I was a kid. I really wish I had pics of those just to show off the creativity, they were no means accurate or professional and didn't last long with a little brother playing with me. LOL
I think that my first prop that I still have on me was a home made Klingon painstick that I carried with me at conventions (back in the early 90's). It really wasn't screen accurate (at the time it was difficult to find screen shots) and since this was to match a costume, it was good enough for pictures (again, back before digital cameras were around).
The notched out part held a 9v battery and circuit board that my father had rigged to the prongs. I recall it didn't work as well as we hoped, but it still scared the crap out of people when it saw them coming towards them. As you can see, it got alot of use. I'm thinking of making a new one with a joy buzzer (wind up or electric) on the tip. But, I'm still thinking on that project and will have to look around the hardware store for more inspiration.
The first "professional" prop replica I made was a Zippo lighter like the one used by the cigarette smoking man. It was a plan $10 Zippo with the text "Trust No One" engraved on it. I did that when I worked for a sign and engraving shop. I used it all the time to hep give it a worn look, but since I stopped smoking years ago, I seemed to have misplaced it. I have the "blueprints" for it somewhere. I will look for both and start their own thread when I do. It bothers me that I can't find it, I was very proud of that lighter.