How can i get the connectivity between my front end (c#_ and back end (sql) please

You can Add Namespace as system.data.sqlclient
and then you can use
sqlconnection class and sqlcommand class to establish
connection with backend sql server

like this

sqlconnection cn = new sqlconnection();
sqlcommand cm = new sqlcommand();

cn.connectionstring = "your connection string goes here";
cm.connection = cn;
cn.open();
cm.commandtext = "your sql query";
sqldataadapter da = new sqldataadapter(cm);
dataset ds = new dataset();
da.fill(ds);
cn.close();

Apart from the answers already given, which will suit the purpose specified in the question, I think this Code Project article
General purpose class to fill DataTable(s) from DataBase and to save DataTable(s) to DataBase using reflection[^]
in which a general purpose class to easily read data from a database and to save data to database is given, may also be helpful.

Try This Mrunmay...

SqlConnection con = new SqlConnection("server=servername;database=databasename;integrated security=SSPI;MultipleActiveResultSets=true");

On Insert Button Click

SqlCommand cmd = new SqlCommand("insert  into employee values('" + textBox1.Text + "','" + textBox2.Text + "','" + textBox3.Text  + "')", con);
con.Open();
cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("Inserted!");

Enjoy Coding