Mobile application

I was assigned to develop a app which will run on all mobile device such as iphone, xoom.
How to start it? What tools do I need?

First of all this is the CodeBank for VB.net code snippets, you are talking about a different platform and different language.
The android apps are written in Java while IOS apps are written in objective C.
Android Provides memory management for apps while in IOS apps must manage their own memory.
If you disregard the operating system you might be able to write code(in almost any language) that compiles to ARM that would be able run on anything with an arm processor, but the Idea that you can write a conventional app that would run on both IOS and Android is impossible to the best of my knowledge.

EDIT: After a bit of research I found that it is possible to run C++ on Iphone and android, but creating one app for both still seems impossible or extremely hard. If you create an app for android and IOS than you covered any mobile device worth having, so that is what I would do.

Moved From The VB.NET CodeBank (which is for sharing code rather than posting questions :) )

You should try html5, check out this link or this one.
Good luck with your project

iPhone Apps can either be built using Flash CS 5 (They added the ability for iOS), or with XCode.
XCode is Mac-Only, and to publish the application you need a developer certifacte "account",
which costs 99$.