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.
To be a good trainer a prerequisite is good communication skills, and in particular effective presentation and public speaking skills. 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. However 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 and I would urge you to focus on that first before embarking on training career. However you must create/ find opportunities to begin your training quest perhaps by addressing social organizations like Rotary/ Lions clubs and also take sessions for college audiences.
To get the academic inputs on training you can do the course from Indian Society for Training and Development (ISTD) as planned. Alternatively you can also attend some Train The Trainer (TTT) programs offered by private institutions.
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.
As mentioned earlier, to make up for the lack of 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 Inspirational and Motivational Blog listed below which can also assist you in mission of becoming an effective trainer
Let me, at the outset congratulate on your decision to become a good soft skills trainer. A, good trainer should have above all, a passion for teaching. Secondly, he must keep his mind's antenna open always and be able to receive all good latest information. He must keep abreast with the latest developments in the field of interest and therefore should be a voracious reader. He must constantly apply his mind to improve his presentation techniques depending upon the composition of the audience and level of their grasp of the subject. He must be a constant innovator of the new techniques and must be able to assess the mood of the audience effectively. He should adopt the 'facilitator' approach where the training will be 'participant centered' and create an effective environment where the participants learn by themselves rather than by 'mere teaching approach'. More questioning techniques and role plays should be included so that participants are made to think and then answer. In role plays they learn by acting and this gives them a 'feel of the things' they have to do. The other techniques include voice modulation, very good, standard and decent body language (I, on my part, like the body language of former cricketer and M.P. Navjyoth Singh Siddu, who is very decent and articulates very well in his vocal expression and another cricketer Suresh Raina, on the field). A lot of eyes will be seeing the facilitator and therefore your body language, expression, language that you use should all be of high standard and exemplary. All these can be acquired by constant practice and developing genuine interest in the technique.
You can also some good books and articles on this subject by making good use of Google search engine. Talking to better experienced trainers and sharing your thoughts and experience and getting their advice will go a long way to make you one of the best trainer in the filed.
I wish you all the best in your future endeavors.