How does the following not do what you want? (The question is vague to me).
string[] MyString = oldstring.Split(':');
if (MyString != null && MyString.Length > 1)
{
MyString[0] = MyString[0].Trim();
}
oldstring = string.Join(":",MyString);
This will split oldstring into an array, trim the first word, then join the array together into a string again. The first word is trimmed, nothing else is touched.
After reading your question again, I'm starting to think that you're confusing what "Trim" means in this case. Trimming means removing whitespace. It looks like you're talking about removing the first word. If that's the case, change MyString[0] = MyString[0].Trim();
to MyString[0] = "";
and run the rest of the code as shown. This will remove the first word of the array and return the rest as a string.