You can either use a typical barista/restaurant style whipped cream can or you can pick up a cracker from a head shop. The whipped cream container is a bit less grippy on the balloon than a proper cracker, but you don't have to worry about your hands getting too cold. The gas is compressed so it comes out very cold.

Hitting them from a balloon is a lot more pleasant than hitting them from a whipped cream compressor, and balloons are absolutely necessary for a cracker or a tank. A tank without a balloon would likely freeze your lungs and kill you. It also facilitates putting an absurd amount of gas into two or three balloons and going through them a bit faster. It doesn't stay in your system long, so you kind of have to work at it to get a decent dose all at once.

The nitrous itself is actually fairly benign, but you have to make sure you take air in with it or you can get light headed and maybe pass out. But that'd happen with any concentrated non-oxygen gas being inhaled in large quantities. So always sit down, and pace yourself.

Doing way too much way too often can lead to this weird wrinkly hand condition apparently, but I've never met anyone who's even come close to doing that much that regularly. Most people I know who've gotten into it either buy a few balloons at shows or pick up a box of carts once in a blue moon. It's incredibly addictive for a very very short time and then really not remotely. Probably why it isn't restricted in the US at all. I've seen it sold at concerts in front of cops many times. And there's definitely no hiding it. You can hear a tank from easily a quarter mile away.