Interfacing microcontroller with GSM modem

I have a GSM USB modem. I want microcontroller to read SMS from the modem and respond according to the specific content of SMS and send back a message to the number with desired content like load condition or meter reading. There should not introduce any computer/PC.

Please help me in this work.

First of all, you really are better off using a modem with a UART or RS232 interface rather than a USB as you don't really need USB speed for a standard GSM modem calling SMS commands, and USB isn't easy to interface to, especially as a host.

Your controller of choice will need a USB host, OTG may be sufficient, but you will need to figure out how to talk to the USB modem.

If you wish to rethink your design with a UART / RS232 interface then there are plenty of threads in this forum and a few open source projects that will help you, just use the search bar.