How to remove all characters from a string before a specific character

string a = "Hello_World";
a = a.Substring(a.IndexOf("_")+1);

try this? or is the A= part in your A=Hello_World included?

You have already received a perfectly fine answer. If you are willing to go one step further, you could wrap up the a.SubString(a.IndexOf('_') + 1) in a robust and flexible extension method:

public static string TrimStartUpToAndIncluding(this string str, char ch)
{
    if (str == null) throw new ArgumentNullException("str");
    int pos = str.IndexOf(ch);
    if (pos >= 0)
    {
        return str.Substring(pos + 1);
    }
    else // the given character does not occur in the string
    {
        return str; // there is nothing to trim; alternatively, return `string.Empty`
    }
}

which you would use like this:

"Hello_World".TrimStartUpToAndIncluding('_') == "World"

string A = "Hello_World";
string str = A.Substring(A.IndexOf('_') + 1);