How can I convert String to Int?

Try this:
int x = Int32.Parse(TextBoxD1.Text);

or better yet:
int x = 0;

Int32.TryParse(TextBoxD1.Text, out x);

Also, since Int32.TryParse returns a bool you can use its return value to make decisions about the results of the parsing attempt:
int x = 0;

if (Int32.TryParse(TextBoxD1.Text, out x))
{
// you know that the parsing attempt
// was successful
}

If you are curious, the difference between Parse and TryParse is best summed up like this:

The TryParse method is like the Parse
method, except the TryParse method
does not throw an exception if the
conversion fails. It eliminates the
need to use exception handling to test
for a FormatException in the event
that s is invalid and cannot be
successfully parsed. – MSDN

Convert.ToInt32( TextBoxD1.Text );

Use this if you feel confident that the contents of the text box is a valid int. A safer option is
int val = 0;
Int32.TryParse( TextBoxD1.Text, out val );

This will provide you with some default value you can use. Int32.TryParse also returns a Boolean value indicating whether it was able to parse or not, so you can even use it as the condition of an if statement.
if( Int32.TryParse( TextBoxD1.Text, out val ){
DoSomething(..);
} else {
HandleBadInput(..);
}