First you need to set-up Port Forwarding in your DSL modem and any routers you have to forward any requests from the internet on port 1433 to your computer, also open port 1433 in any firewalls, then you need to give Clint your public IP address (assigned by your ISP, which may change if your modem resets, if your ISP doesn't assign static addresses).
Connection string becomes;
Data Source=Your Public IP Address,1433;Network Library=DBMSSOCN;Initial Catalog=YouDataBase;User ID=myUsername;Password=myPassword;