We are newly married and as a couple should we look to start our own business as a career goal? if so what percent of our earning should be focus to save so we start our own?

You must come out with a buisness plan 1st.

How do start? Source for possible vendor? How to find customer? Back up plan?

The most important budget. You need to calculate how much $$$ you need, and if no buisness how long can you last, becoz starting may be slow. If you are comfortable then can start.

It depends on how much you have every month. You can set aside a certain % that you are comfortable with.