This is a major issue for especially, but not limited to, amateurs.
There are a few things to remember if you want to gain confidence:
1. Remember: no one thinks you're an idiot.
What do I mean by this? One thing that makes people really nervous is that they think that if they mess up a trick, people will hold it against them. A slight mistake and it gets beginners and some professionals very scared. If you mess up for a group, just go on to another trick after blowing it off as a joke. PEOPLE WILL NOT HOLD IT AGAINST YOU.
2. Laymen don't know the tricks or slights involved.
When you perform for some one, and you make the slightest error or a small flash, don't say, "Shoot, this messed up" or even worry about it. Ninety nine times out of a hundred, people on the street will not notice it.
3. Build up your self esteem.
Go to a bowling alley or park, etc. I really recommend this. It is so easy to get people to agree to watch a trick if they are relaxed. If you do this many times, you will be much more excited than nervous and maybe not nervous at all. I am not nervous because of a bowling alley. Her's my story: I watched the score board of the people in the lane next to me so I Learned one guy's name, let's call him Jake. After his turn, I went over to him and said, "Oh hey Jake, long time no see." He looked at me like, who the heck are you? but I just said, 'Remember me- Yehuda that magician? No. Well actually I just saw your name up there but I actually am a magician and are you interested in seeing a quick card trick?" He said yes and that's why I now become more nervous asking for help in a store than offering to show a trick to a complete stranger.
I hope this was helpful. Please respond.