At the outset, let me compliment you on deciding to be a trainer. As a person in sales, I can safely assume that your communication skills are excellent because that is critical to becoming a trainer. However, the heart and brains of the training still remain the domain knowledge, but even the most knowledgeable person would fail miserably as a trainer without effective communication skills. If you are a good communicator, particularly a good public speaker, half your problem is solved. To further improve your communication skills, find a local Toastmasters club in your city and join them.
To get the academic inputs on training, you can do the course from the Indian Society for Training and Development (ISTD), which you are already contemplating. Alternatively, you can also attend some Train The Trainer (TTT) programs offered by private institutions. One of the institutions has already informed you about their program in the preceding reply.
To get going, focus on those topics that you are familiar with, confident about, passionate about, or have a good grounding in. Keep updating yourself by reading extensively and start outlining the broad course contents, keeping in mind the type of audience you are likely to address. In particular, pay attention to the type of exercises, examples, and activities that you will use during the program to reinforce a point or make the learning relevant. I would also suggest that you attend some corporate training by professionals on topics of your interest to get a feel of how it is structured and delivered.
If you have no corporate training experience, ideally take a few sessions for postgraduate students in colleges or for social service clubs like Lions and Rotary's so that you gain experience and can fine-tune your material. Once you are confident, approach a few corporates, and the first few training programs are vital for future reference and for your confidence.
I would also invite you to visit the following Inspirational and Motivational Blogs, which can also assist you in improving your vocabulary and communication skills:
- [Actspot's Blog](http://www.actspot.wordpress.com) - Over 85,000 views and 575 followers
- [Academy for Creative Training](http://www.poweract.blogspot.com) - Over 25,000 views and 160 followers
Best Wishes