At the outset let me compliment you on deciding to be a trainer and motivational speaker. 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. 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), New Delhi. Alternatively you can also attend some Train The Trainer (TTT) programs offered by private institutions. It would also help if you work with or at least attend a few programs by senior trainers.
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 your quest to be an effective trainer
www.actspot.wordpress.com - Over 1,94,000 views and 1040 followers
www.poweract.blogspot.com - Over 59,500 views and 230 followers