Hello,
You can use USART communication for transmitting this AT commands AT+CMGR (read message), AT+CMGD (delete message), AT+CMGS (to send message), please refer the AT commands for more details about syntax to send or receive.
From the microcontroller, Through USART you need to transmit the command with transmission buffer of USART. before that you need to configure which USART you are using in controller and whether the USART is to be in send mode or recieve mode and baud rate to have sychronization b/w the microcontroller and GSM modem.