Everyone has a calling and that person is generally aware of that. It is best that one follows what his or her heart tells him or her when starting the business. As the person has an interest in that field of work, it generally works out best.
The field in which you are educated and trained is generally a good idea. If you have sought education in the field that you are personally interested in, it is the best idea for your business. However, if you interests and training are in different areas, it is a good idea to work in the field where you are trained.
Do consider the financial aspects of starting a business as well. There are many businesses that fail because of lack of liquidity. Make sure that whichever business you set up, you do have enough financial resources to push it through the start, which is often the hardest time.
Study the market and see which business is in demand. It is possible that you may have the training and interest along with the right amount of money needed for the business but not enough consumers. So ensure that the business you take up has enough clientele for it to be sustainable and profitable in the longer run.
You also need to look at the future prospects of the business you are starting. It is important that the business can grow over a period of time and does not slow down altogether after its start. It is imperative for the survival of any business that it remains relative over a longer period of time.