You can change yoru string variable case using ToLower() or ToUpper() method.

s.ToLower();

ToLower() method change characters into lower cases. But ToLower method uses your system's current regional settings. And if your culture settings adjusted for Turkish, for a string variable with "INDIGO" value the result will be "indigo". Therefore you must use System.Globalization.CultureInfo class to adjust for the desirable culture.

s.ToLower(New CultureInfo("en-US"));

You can dicover all the culture names on the link below.
http://msdn2.microsoft.com/en-us/library/system.globalization.cultureinfo(VS.80).aspx
You can use s.ToUpper() and s.ToUpper(New CultureInfo("en-US")) to change character cases into lower.
Türkçe'si için tiklayin