If you are known at all as any kind of photographer by your friends, eventually you'll be asked to shoot a wedding (even if your usual specialty is unrelated-- for instance, medical x-rays). I've only shot a few, and am not very good at it (people with clothes are hard to pose!), but here's a few thoughts (and there are many and better pieces of advice to be had from more experienced wedding photographers, so ask the experts!).
My first suggestion is not to do it. It's not worth the tiny amount of money they think you should be paid, and it could easily ruin your friendship.
Or if you must, to shoot a few friends' weddings for free (with some other paid photographer handling the real stress- and don't get in their way or shoot over their shoulder!) until you have some experience. Or shoot a wedding for friends who can't afford a pro and will have nothing but "relative snaps" without your help. A few points:

1. Churches are terrible to shoot in--often too big for flashes to work, and some don't allow flash photography (or any photography) at certain points in the ceremony. If you don't have a good camera and a separate handheld flash/reflector, etc., you may have problems. Check the rules first, and remember you have to work FAST. Be prepared to change lenses to get some closeups, some long shots with the windows behind them and the altar, the aisle, etc. Meet with the bride and groom beforehand and see what they have in mind for ceremony pictures. Make them show you examples.

2. Brides and grooms are insanely busy and stressed, and will forget things you need, like time. I once showed up 20 min before the time I had been told to arrive at a church, only to meet a frantic groom in the parking lot- they were all waiting for ME so they could walk down the aisle! They had told me the time of the actual vows, not an hour before the ceremony started so I could set up, figure out angles, check exposures, etc. It was a nightmare. Be there before anyone else, with extra batteries, data cards, and a backup camera. Have an assistant for reflectors, flashes, herding (see below).

3. Herding family members into the required six thousand combination group shots is like herding cats- the bride's mother is tired and grumpy, the groom's father has already left, the bridesmaids are getting drunk somewhere. If you don't know what people together in which shots are important to the bride & groom beforehand, you WILL miss stuff they want. You might think you're just there to take pictures of the happy couple, but their Great-Aunt Ethel came all the way from Australia and they want her in the family photos, etc. Add to that the fact that they will have forgotten to put aside any time for posing between ceremony and reception, and if the church isn't near a nice outdoor space, you're screwed.

4. After the ceremony: their expectations of turnaround time (as you desperately try to find pictures where everyone looks human and edit them quickly), and price for prints, etc., is often completely unrealistic- and if they're away on honeymoon you have nobody to talk to about which shots are worth editing, etc. They may not understand that "printing off a dozen of those" is a very expensive proposition, or that you may not own the equipment to do it.

As you might have guessed I hate shooting weddings, and will only do it for friends, for free. That way they understand clearly that I am not making any promises, and if they want a professional who they can yell at if they're not happy, they should hire one. I'll do what I can and edit when I have a chance, and that's all. I shot my best friend's wedding and the card died before I'd had a chance to back them up, losing everything. I felt terrible. If I hadn't discovered the German data card recovery masters, Recoverfab (best $220 I ever spent), the whole wedding would have been lost.
I was fortunate enough to have a kind friend and excellent art photographer shoot my own wedding as a gift, and the results were great- because he's experienced, and has helped teach wedding photography workshops before (including one I modeled for).