Hi Simon,
Do have a read of past threads on the subject.
The biggest thing you must do, and start now, is to set their expectations at a realistic level.
On a more personal level, can you set yourself some targets to slowly get yourself used to shooting in crowds?
Start small and work up gradually, it is only about building confidence - you can do it, you just don't know it yet
e.g. not sure whether my uk based suggestions will help (where-ever you are), but;
start at a small tourist attraction, where there are a few others with cameras
~ insert other ideas here ~
move on to say a village fete
~ insert other ideas here ~
observe a wedding at the same church(?) or registry office(?)
Hope that helps and welcome to the CiC forums from ...