You did not say which GPS module you are using..but on the one I have I can set the message format to GGA,VTG and RMC..
I did the following:
make a structure that fitted the RMC message format,then waited for the start char $ (24h), then put the incoming data into the structure item until the comma arrived (2cH),then dropped to the next item in the structure until all the data was stored.