Extracting first name, last name, phone number, and address from a form
You could create a method for it!

bool Validate(string val, TextBox textBox, string description)
{
    if (!string.IsNullOrWhiteSpace(val))
    {
        val = textBox.Text;
    }
    else
    {
        MessageBox.Show("Please enter " + description");
        return false;
    }

    return true;
}


Then you could use it like so:

if (!Validate(firstName, firstNameBox, "a first name"))
   return;
if (!Validate(lastName, lastNameBox, "a last name"))
   return;