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 a critical to become a trainer. However, the heart and brains of the training still remains the domain knowledge but even the most knowledgeable person would fail miserably as a trainer without effective communications skills. If you are 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 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 exercise, examples, 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 post graduate students in colleges or for social service clubs like Lions and Rotary's so that you gain in experience and also 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 own 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 - Over 85,000 views and 575 followers
Academy for Creative Training - Over 25,000 views and 160 followers
27th October 2011 From India, Mumbai