where to start
Start by sitting down and deciding how much money you want to make per day after tax and expenses. This kind of thing is easy to do with tractors - things with small tractors need to earn £100 an hour, and big tractors £150+ an hour to pay the wages, machine etc.
To be realistic, so your mates who are teachers and so have loads of holiday and do no real work dont "skin" you with better cars / houses etc for your whole life, you want to clear £100 a day in profit after tax and expenses or you may as well go work in Tesco and trim some sheep in your spring break or at weekends.
Secondly, is there enough work to keep you busy for a year? Trimming, shearing etc is not exactly going to cost you a lot to set up, so can you add other things for sheep people? How about fencing too? Hedge laying? Otherwise you are going to be fairly quiet in wintertime. There is certainly call for the shearing but it isnt enough for a business.
How to get customers? Ask some vets. They are often at the business end of a sheep, so can spread the word so to speak.
Not for print please.