As in any other industry, you can enter the training field by purchasing a training franchise from a major existing company. Basically it works like any other franchising system.
You pay a sum of money, often in the $30k and up range. You receive the rights to offer training or coaching or consulting services that have been created by the parent company. You may receive an exclusive territory, which can be a state or province, or some other regional area. You will probably receive some basic training in the specific products and services you will be offering/selling.
Marketing is up to you, of course, as are any other startup costs.
Buying a training franchise tends to appeal to those with limited training expertise and experience, and seems to attract people who vary in terms of their abilities to succeed in their business. It is a very expensive way to enter the training field.