Let me compliment you on deciding to be a trainer and would like to share my suggestions for your consideration.
Importance of Communication Skills
To be a good trainer, an important prerequisite is good communication skills, particularly effective presentation and public speaking skills. 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. To further improve your communication skills, find a local Toastmasters club in your city and join them.
Academic Inputs and Training Programs
To get the academic inputs on training, you can do the course from the Indian Society for Training and Development (ISTD) as planned. Alternatively, you can also attend some Train The Trainer (TTT) programs offered by private institutions.
Focusing on Familiar Topics
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.
Gaining Experience
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 as the first few training programs are vital for future reference and for your confidence.
Inspirational and Motivational Resources
I would also invite you to visit the following Inspirational and Motivational Blogs which can also assist you in your quest to be a good trainer:
www.actspot.wordpress.com - Over 1,14,000 views and 710 followers
www.poweract.blogspot.com - Over 35,000 views and 190 followers
Best Wishes