First thing would be more detail on the sensor and more detail about what you are trying to do. What do you mean 'control GSM and LCD according to' the sensor? Are you trying to display a fuel level? Are you trying to send an alarm message?
To get you started, I'm going to guess the sensor is resistive with float, as they are quite common, or potentiometer with float, but there are other options. If it is, then the easiest way to read it is set up a voltage divider, one end to ground, the other to your processor supply, and the wiper to one of the analog capable inputs, and read the analog value using the ADC. The sample for reading the internal temperature of the MSP430 (see the LP documentation) is a good starting point for using the ADC, and there are a number of examples here in the archives, as well as lots of examples in the TI lit and around the web.
