Basically, you need a gear that is easy to pedal in, but won't kick the bike out too much when you put a bit of pressure on it. I normally use something around a 32:16, or 52 inches (on my mtb). As for the slope required, once you master trackstanding, you sould be able to do one on any slope - I can now do one on a downhill (ie using the brakes).
Unfortunately, there is only one way to learn - and that is to practice. I'm talking about a good 1/2 to 1 hour a day, not practicing only when you get to traffic lights. Trackstands are like wheelies - some people can do them easily, most people have to practice lots to do one. Keep at it, and eventually you should be able to do them no worries
Oh, and yes - people will laugh at you if you initiate a 'horizontal trackstand'