The most likely platform for your program is Microsoft Windows, because that's where most Graphical User Interface ( GUI ) applications run.
In that case, I would design it in Windows Presentation Foundation ( WPF ), using Microsoft Visual Studio. You should choose a language type of C#, not Visual Basic.NET.
If you use WPF, you will be ready to immediately code the rest of the application if it goes to that stage.
You can get a free development environment here for C# for Windows:
Microsoft Visual C# 2010 Express
http://www.microsoft.com/express/Windows/
This may allow WPF projects. If not, you would need a full version of Visual Studio.
WPF is Microsoft's strategic platform for GUIs on Windows PCs. Web is separate and might be Silverlight ( based on WPF ) or HTML 5.
If you are designing a web site, the answers would be different, but you said 'program', so I am assuming a PC-based application.
There are other tools like Microsoft Expression Blend and Expression Design that you might want to evaluate as well.
At the end of the day, you have to make your own decision, and what I recommend based on my experience may not be suitable for you.