Find the right com-Port. Go to control panel, telephone and modem, modems. Here you have your com-port. For the baud rate go to Settings and Modem after selecting the correct modem from the list.
AT+CSCS="GSM" // change character set to GSM
AT+CUSD=1,"*101#",15 // Issued balance check ussd code, to refuel your account, simply replace '*101#' with the corresponding 'refuel number'