// note the usage of the single quotes;
string insertString = @"insert into tbl1 (Col1, Col2, Col3) values ('" + txtField1.Text + "', " + chkBox1.Checked + ", '" + txtField2.Text + "')";
// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand(insertString, conn);
// 2. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
// note the usage of the single quotes;
string insertString = @"
insert into Categories
(Col1, Col2, Col3)
values ('" + txtField1.Text + "', " + chkBox1.Checked + ", '" + txtField2.Text + "')";
// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand(insertString, conn);
// 2. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
For a better understanding please read:
http://www.csharp-station.com/Tutorials/AdoDotNet/Lesson01.aspx