i would suggest first of all gauging the level of interest - perhaps a survey, or an ad in your local paper asking people to contact you if they would want your services, because you may find there is simply not enough people interested in your vicinity to sustain a business, which will mean you will have to aim further afield - which could involve a lot of travel, having your car made safe for animals, learning the local parks and footpaths etc - and you still may find a lack of interest.
i would think many people would not want a stranger in their house to watch their animal, so you would also have to consider having all the animals in your own home - which would involve you swotting up on the behaviours of various animals - which dogs will fight, attack etc, other animals, how to calm nervous ones etc.
you will need an area just for them, with toys, bowls, bedding etc, unless you want to let them run wild in your house.
you will probably need a back yard.
you will need some type of basic veterinary knowledge for when the animals are unwell, and may need medicines administering or dressing changed etc.
i would think sick animals is one of the main reasons people need a sitter.
you may need an occasional assistant for when you are unexpectedly needed to look after an animal full time, but need to check on and feed others, because some will get into a routine and used to your visits
you may find some people want to see a crb check on you - you can buy one from your local police station for �10.
i would suggest getting some flyers printed - or an ad in a paper, PO window etc, and just see the responses you get.
if you have a local free paper , you can get the flyers put inside the paper and they will be delivered with it.
if it looks to be popular, then you can start on all the other stuff.
i would suggest a lot of research of others doing this