Oh! I've been in this situation many many many times. What I'm about to say has probably been mentioned but I didn't read the whole thread. Here is what I would say (also sorry if some of this is simplistic, I have no idea what your skill level is):
-Make sure up front they know that this is your first wedding, and you recommend hiring someone, and you can't guarantee awesome wedding photos. This will (hopefully) get any assumptions about superb quality out of their heads. It's really important that their expectations are realistic regarding the quality of the pictures.
-Give them a realistic turn around time for photo processing and don't give away any work until its processed. Ever....
-Shoot raw. Lots of times at weddings you run into mixed lighting and raw is by FAR the easiest way to processes it.
-Get the most out of this opportunity and use it as a learning experience. Splurge a bit if you can on some new gear and get comfortable with it.
-Get a lot of SD cards, you will need them.
-Make sure to have extra batteries.
-Get an external flash, learn it, and be comfortable with balancing flash and ambient light. This is a crucial skill for wedding photography.
-Plan the shots. Work with the bride on the shots she wants. Detail shots (brides dress, wedding invitations, shots of the music being played) are just as important for a lot of people as the people shots. Carry a little checklist with you and get 'em all. Despite what the bride says up front, she will always want more.
-Do some engagement shots this spring. This gives you a chance to work with the couple, take some pictures, etc.
-Practice low light shooting. Scope out the Church they are getting married in before hand and see if you can practice shooting in it. Make sure you know the rules for flash photography. Some preachers are very selfish and either A. Don't let pictures at all during the ceremony or B. Don't allow flash photography from the wedding photographer but don't seem to care about uncle bob in the front pew.
No flash during the ceremony is pretty common but like I said, be prepared because some selfish preachers won't let any pictures during the ceremony. The bride needs to be aware of this.
-Rent some fast primes. Something with reach like an 85mm or 135. 1.8 or 2 aperture. These will be VERY useful.
-Practice practice practice! Shoot plenty of portraits and detail shots. Your skills will drastically improve. Make sure to post them online for critique. Make sure you take a lot of low light shots. Most of your wedding will be in low light. Also make sure you can get nice bokeh in your shots. Believe it or not just a little bokeh is all that stands between a bland and an outstanding photo.
I could go on and on and muck you down with details of getting a second body, or how to bounce flash, etc (a demb flip-it is currently my favorite for bouncing flash but aside from that) I will leave you with those basics above.
Weddings are EXTREMELY stressful and the pressure is always on the photog to get great shots. No matter how good your shots are brides always want more.
Also be careful where you post that you arent charging for shots. Pro wedding photogs will ream you on forums for that (fredmiranda.com is an example).
I've shot many weddings but I do not do it professionally. Two of the ones I shot were for friends because the wedding photog cancelled a week, or in the last case 12 hours before the wedding and I was the only photographer friend of the couple. Its hard to say no to a bride who shows up at your apartment at 2am of her wedding day, tears streaming down her face cause her photog cancelled. Luckily I had the gear and was prepared. All other weddings I shot were assisting friends who needed an extra hand (or bit off more then they could chew
)
In the end it is a great learning experience and who knows, you may end up loving it.
Anyway GOOD LUCK! Make sure you keep us posted