Alis given some good advise there. I have been a wedding tog for more years than I care to mention, theres a lot more to the job than just buying a camera and away you go.
You need to market your business, remember if your just starting theres a long lead time with weddings (most will be for next year or later) so not much money throught he door for a while.
You need to be a people person, not just the life and soul of the party, you have to manage and control maybe a couple of hundred people, without upsetting them.
You have to know your equitment inside out, what it will or wont do, how to get around the things it won't do. You have to have back up cameras, lens, flashes etc etc, the 450d is a good camera, but it's not made for hard use, you might want to look at a 40 - 50d ofr even a 5d.
You have to be able to deal with the unexpected, stuff does happen at weddings, I have had people die, people fighting, you name it and it'll have happened to some poor wedding tog.
Then theres the business side, keeping accounts, pricing your services correctly, paying the tax/vat man, having insurance, third party and camera. And how will you manage if you break your leg just before a wedding? have you got another experienced tog who will "stand in" for you.
Theres a million and one things to think about, if your asking about how to use the camera properly your shouldn't really be even thinking about doing a wedding yet, screw up someones special day and you'll be lucky if you only get sued, you'll not get many more bookings once word goes around.
I'm not trying to put you off (your no threat to my business where you are) but I'd hate to see you get yourself into a mess. learn the camera totally and ask again. Wayne