developing a mobile app

Hi Guys

we are planing to design a mobile app?
i need resources about this topic?
like
difference betwen a mobile app and a link to a web page in my mobile screen?
how hard is to customize a website to be mobile friendly?
How does the plattform affects the price?
if i need an app for iphone, apad, droid, windows mobile, etc? are these different prices or one package?
can i send text messages to users that are using my app?

Is there any tool to help create a mobile app?

If i decide to do just a link to my website as an icon, how can i distribute the icon, or users have to create it?

We have 2 phases for this mobile app
1.- We have a database online that we would like to be accesible by a mobile device?
So i need to cusomize my site to eb mobile friendly or create a new mobile site?

2.- I would like send them reminders by text, send them feeds, live support option, etc

what are my options

Where do i start

Please help

thanks