Mobile Iphone Android Task Calendar Support Site

I have a collaboration tools site that is almost done... I want to develop a iphone android version of the basic features of it, and later improve the functionalities. User log in to his group and personal account, check his calendar, read/send tasks, read/register support tickets. This for the basic version I want now. Later, some document management features, etc.. As this is my first experience with mobile, I want guidance about what can be done within the budget and, if first step goes OK, plan future developments.