The kind of questions you are asking are a bit worrying, coz you need to be familiar with the basics and operating your camera, in order to "think on the fly" as the situation changes. If you're still struggling with "errr... what mode to use?", then the moment will be lost before you know it.
Make sure the wedding couple are fully aware that this is your 1st/2nd time, and set their expectations right.
Then go practise and be prepared, and try to leave them pleasantly surprised!