<p>How does one start development in Silverlight?</p>

<p>Does one need a new IDE? or Visual studio will support?</p>

Yes there is tooling support for Visual Studio. It is still in Beta though.
Get Started Building Silverlight 2 Applications
1) Install Visual Studio 2008 then Install Silverlight Tools Beta 2 for Visual Studio 2008
This add-on to Visual Studio 2008 allows you to use .NET to create Silverlight 2 Beta 2 Web sites. The Silverlight 2 Beta 2 Runtime and the Silverlight 2 Beta 2 SDK are installed as part of this install. For additional information read the overview and the Silverlight 2 Beta 2 Readme Notes. Note if you have Visual Studio 2008 Service Pack 1 Beta installed, please see this information for details on installing correctly.
2) Install Expression Blend 2.5 June 2008 Preview
This is a preview version of Expression Blend for designing Silverlight 2 experiences.
3) Install Deep Zoom Composer
This tool allows you to prepare your images for use with the Deep Zoom feature in Silverlight 2.
One thing to watch out for is that Silverlight does not support Synchronous calls to the server. All calls are
Asynchronous
as of
this Beta
.