You'll want to find out the laws in your area about launching them. In the US, you can launch them freely without necessarily notifying anyone as long as they'r

You'll want to find out the laws in your area about launching them. In the US, you can launch them freely without necessarily notifying anyone as long as they're below a certain weight and meet other requirements (maximum breaking strength of the line used to attach the balloon to payload, using a radar reflector, etc.) However, it's still HIGHLY RECOMMENDED that you call and let any airports know if you're launching close to one. Look into APRS and how to build (or you can purchase one) an APRS GPS transmitter, that'll be the most important bit as if that isn't working, you'll very likely never find your balloon. Once that's working, you can use a site like aprs.fi to track the balloon in realtime from a laptop/mobile device and follow it as it flies. Keep in mind it could VERY well travel over a hundred miles. We launched ours from Decorah, Iowa and we had legitimate concerns about wind patterns causing it to fly as far as Chicago. I'd also recommend looking into how to build a "Fox" and how to foxhunt. That's a big part of these balloon launches too! Once you've gotten as close as you can with GPS (once the balloon falls below a certain altitude, relays will stop picking it up on APRS so you'll have only a general idea of where it fell), you'll want to use a directional (for instance, a Yagi-Uda) antenna and a handheld radio to try to directionally locate the fallen payload. Funnily enough for us, we didn't have to foxhunt since it landed in the middle of a guy's field, when we parked across from his house, he asked what we were up to - he was friendly and just curious - and we told him, he said he thought he saw something shiny in his field and assumed it was garbage blown over from the road or something and let us come over to look at it. Turns out it was our balloon! The shiny thing was the radar reflector. Then, put whatever you want in your payload really! We used a paint can stuffed with foam (it gets REAL cold up there, like -100F, keep that in mind!) and shoved sensors into it and attached a 360 gopro to the side. You can find stuff about how to build temperature/humidity/etc data logging setups that log to an SD card. I'd also recommend that after you retreive your balloon, go to some place like aprs.fi and download the GPS track data from the launch! Those sites don't keep it around forever, you'll definitely want a copy for yourself. You can get the balloons themselves quite easily, and welding gas suppliers should be able to get you a cylinder of helium. Good luck!