Hi Ying Ying,
The easiest and fairest way to do this is to implement a dynamic equity split. A dynamic split, unlike a more common fixed split which forces you to guess about future inputs, allocates equity on a rolling basis based on the actual contributions of time, money, ideas, relationships and other resources contributed by team members.
I've developed a model for implementing a dynamic equity split called the Slicing Pie method. The model has two pieces. First is the allocation framework that outlines how to slice up the pie among founders. It converts all possible inputs to slices. A person's share is equal to the number of slices they contribute divided by the total slices contributed by everyone.
The calculations used to determine the slices is based on the fair market value of the contribution and some risk multipliers.
The next piece of the model is the recovery framework which outlines what happens in case somebody leaves the company. Whether a person keeps slices depends a lot on the circumstances under which they leave. Getting fired for cause, for instance, is different than getting laid off due to budget cuts.
There are a number of places you can go to get more detail. I recently wrote an article on this site that explains more: http://founderdating.com/formula-for-the-perfect-cofounder-equity-split/
Or you can go to my web site, www.SlicingPie.com. After taking a look feel free to contact me if you have questions.