I often find myself in various parts of the world invited to speak in my particular areas of expertise - I make a point of singing a relevant song at the start because I think it's important .... oh, for a lot of reasons. However I don't consider that 'performing' as the song itself is not the point, my thoughts and research in my areas of expertise are. Recently I thought I would overcome fearful performance anxiety and that very vexing question that you have raised, how do you start, by attending a session in someones home and giving it a go. I had my songs ready to go. I was gonna do it!! I passed the first circle as I thought it better to let the session get into gear a bit. However, after the first circle everyone just started doing their own songs, including a couple of choirs who really got into it. My chance was lost! The lesson is, and as everyone else is saying, just get in at ANY opportunity, and do it. One important point has been raised which is the one about doing stuff for free. Do that only where you are not doing a musician out of a job. If you dither, as I did, you'll lose the opportunity and that's one less experience and one less round of confidence you have.