You're in for a long journey. You first need to specify what you mean by variable voltage, variable frequency and what your input will be. And if this is a first project, you have chosen one that couldn't be harder. If this is some thing you need right away your better off buying one from a company that makes them. If just something to learn from, Google for design papers on it and read, then read some more.
As shortbus says, a lot more info is needed.
What is the range of voltage and frequency? Good answers need good questions, so, have another go at asking what you need.