Hi, missmarylou,
I've only been in the wedding photography business for a short while (since 1971 and over 500 ceremonies) so what do I know? One thing is this: if I had a dollar for all the brides who cried on my shoulder for having their inexperienced friend or 'uncle Harry' shoot their wedding, I'd be financially independent by now. Well, almost. But you get the point.
And, OMG! Anybody who would recommend that you treat a church ceremony like any other photoshoot by so much as suggesting the use of flash is not thinking about (a) the distraction AWAY FROM what's really important - the ceremony and (b) overriding the normally beautiful lighting that is characteristic of most churches.
There are better ways to 'learn somehow' and better other places to be 'bold and mighty.' A friend's wedding is not one of them. One is to read a book on wedding photography by a seasoned pro. (I, a seasoned pro, bought Digital Wedding Photography by Glen Johnson and learned a lot!) Another is to find a local seasoned pro and attach yourself to him or her. It's how I started, it's what I still do, so I still take the same medicine I'm prescribing.
WE'RE TALKING ABOUT A BRIDE'S WEDDING MEMORIES FOR YEARS TO COME, FERCRYIN'OUTLOUD! If you value your friend, don't use her to advance yourself at her expense.
But..... if you're going to do what I think you're going to do anyway... dress in a way that shows respect for the couple, the occasion and the house of worship; stay behind the last occupied pew (the guests didn't come to watch your backside); shoot at around 800 or 1600 ISO (nobody ever orders over 8x10 of the ceremony anyhow); use an image stabilized lens and shoot like an Army sniper (a whole 'nother topic)... and... attend the rehearsal so you know what happens when.
Sorry, kiddo, we've only scratched the surface on a topic deeper than you can imagine at this time. Good luck.
At this point I'm sure I haven't changed anyone's mind, but I have a clear conscience knowing I've done the right thing by urging you to get some training through books and as a 2nd shooter.