It normally starts with someone asking if you could do a few shots at their wedding. This is where you should be saying "No, I hate getting under the feet of the official photographer." You'll probably find out that you are the official photographer, and that's a good time to run in the general direction of away.
Firstly, you could mess it up, and then you'll never hear the last of it. If, like me, you shoot landscapes, you probably regularly mess up, shrug, move on and forget it. Landscapes do not come back and complain the photo is unflattering. Here's a nice landscape screwup - while I was shooting I exhaled, and since I happened to be smoking a fag at the time... Oh well, next time, eh?
Secondly, it isn't that easy. Look at the photo I've headlined with. Even someone who is completely indifferent to photography can see that it looks rather "professional" (which is not the same as "good"). The reason is that it has Shallow Depth Of Field, as discussed in here before. The bride is in focus, the background isn't. Doing that is both expensive and hard (you need a fast lens, accurate focus, and a good feel for how the apertures will work out). Hence professionals like to use DoF effects, because they can, and now everyone "knows" that is how a good photo looks (even if they don't realise what it is that makes it so).
The second bit that is hard is managing the light. The wedding probably won't take place in ideal conditions. That confetti shot was taken in hard midday light, so the only way to stop everybody looking dreadful was fill in flash. Again, you need to know what you are doing. I had no idea (I use fill-in flash a lot, but not on people), but I got lucky.
Thirdly, you probably want to know something about weddings, so you know who you are supposed to be photographing and when. I know nothing about weddings, it has always seemed to me easier to find someone who hates you and buy them a house. Fortunately in this case the groom actually is a wedding photographer, so he was able to help. And we still made a cods of it.
Fourthly, the bride and groom probably don't know all this, which is why they won't stump for a real shooter, so they will be disappointed when you don't do a professional job...
Fifthly, if you are used to doing landscapes, you Post Process. When you hand your thousand shots over (and you should take at least a thousand shots, because at least one person will have their eyes closed, and another will be picking his nose, so make sure you have lots of cards and batteries) and tell the bride and groom to pick forty shots, they will come back asking for two hundred. I don't know what a real pro does in those circumstances, presumably takes them outside for a proper kicking. What I did was PP two hundred shots. At ten minutes each. Which is two thousand minutes. Which is days of work. These modern times, there is a real danger the divorce will have happened before the wedding album has been assembled.
Anyway, it was good fun, they seem to like the shots, and I've done it now.
But I'm not doing another. Ever.
And if you have any sense, neither will you.