At the outset, let me compliment you on deciding to be a trainer. I am taking the liberty of adding my suggestions for your consideration.
Communication Skills for Trainers
To be a good trainer, a 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. Even the most knowledgeable person would fail miserably as a trainer without effective communication skills. Remember that in training, your objective is not to outsmart or outwit the participants but to take them along. Ideally, an MBA from a good institution and work experience would dramatically enhance your skill and effectiveness as a trainer. I would urge you to focus on that first before embarking on a training career. However, you must create/find opportunities to begin your training quest, perhaps by addressing social organizations like Rotary/Lions clubs and also taking sessions for college audiences.
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.
Focus 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
As mentioned earlier, to make up for the lack of 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 own confidence.
Inspirational and Motivational Resources
I would also invite you to visit the following Inspirational and Motivational Blogs, which can also assist you in becoming an effective trainer:
www.actspot.wordpress.com
www.poweract.blogspot.com
Best Wishes