First of all you have to know what sort of business you can do - and whether you have the knowledge to do this. You have to work out whether there is a demand for your service. You need to know where the business will be sited and how much that will cost, plus running costs. Do not forget advertising costs. You will need money to get your business running so you need to know how much this will cost and how much you need to live on on top of this. You need to get the finance for this. You have to work out the legal matters too. Also you need to work out what happens if you are sick or go on holiday.
Firstly you need to know what business is to your liking and and what is the current trend in the market, need to do a bit of research on it, then need to check your resources how much you willing to invest, research what could be your returns, check for suitable locations for your business, lookout for any competitors in the area, keep in mind the draw backs and be prepared to take the risk if you fail, hope that helps