You'll need a transit van (hire one if necessary to save on start up costs). Ask your local council or the DVLA for information about obtaining a passenger licence. Also consult your insurance company and a solicitor about getting the necessary insurance cover.
Next, you can begin targeting prospective customers. Consider nurseries, retirement homes, office groups, community organisations, social clubs, colleges and youth clubs, disabled groups, large families - the possibilities are endless.
Offer customers a block booking discount if they can fill all the seats in your transport. Another effective sales incentive is to offer a free seat to someone booking, say, ten or more customers.
As well as contacting organisations directly, you could try putting up posters in shop windows and community centres, leaving leaflets in tourist information offices and gift shops, and placing ads in the local paper and tourism/travel guides.