How to retrieve data from a SQL Server database in C#?

I have a database table with 3 columns firstname, Lastname and age. In my C# Windows application I have 3 textboxes called textbox1... I made my connectivity to my SQL Server using this code:

SqlConnection con = new SqlConnection("Data Source = .;
                                       Initial Catalog = domain;
                                       Integrated Security = True");
con.Open();
SqlCommand cmd = new SqlCommand("Select * from tablename", con);

I'd like to get values from my database; if I give a value in textbox1 it has to match the values in the database and retrieve other details to the corresponding textboxes.

I tried this method but it's not working:

cmd.CommandText = "select * from tablename where firstname = '" + textBox1.Text + "' ";

How can I do it to retrieve all the other values to the textboxes?