More Than You Think!
A fair number of new training and seminar businesses fail (just like other small businesses) because the owner underestimates the range and depth of skills needed for long term success. So the first answer is, you will need more skills than you think, OR, have the money to purchase the services of people who have the skills you lack!
What are the essential skills? We can divide them into three categories, each of which is important. First, business skills, which encompasses all of the skills that are essential to run, market and manage any business. If you have worked as a trainer for a number of years as an employee, but not an owner, you probably lack at least some of those. Probably the most important is an understanding of marketing.
Second are training process skills. These include training delivery skills, and in most cases, skills in instructional design.
The third category of skills has to do with expertise in the content areas in which you will be offering training and seminars. If you want to offer courses in communication, let's say, you need to be as expert in the subject matter as possible. You can't know "just" what you include in your course content. Your knowlede levels need to be of wide breadth and deep depth.
If you are lacking, it's best to get your skills in place before hanging out your shingle. It's expensive to rely only on learn as you go, and the more you have to learn after you hang out your shingle, the more money you have to have to have to cushion the period when you are learning.
Tip: We all tend to overvalue our own expertise, particularly if we haven't "tested" our knowledge. Take the position that you need to learn more. Always. But particularly before you leave your current job and hang out your shingle.