New to GPS ? (the preferred term these days is “GNSS” include all the different satellite systems).
There are any number of web-based educational resources which can teach you about how it all works. The presentation depth ranges from very solid descriptions using almost no math, to those with more esoteric details and lots of math. Select one you are comfortable with. Here are just a few that we feel are very noteworthy.
Several Educational Institutions also provide good overview briefings as well
Need the details? Get the official message and interface specifications. Here are links to the more basic ones. Some of these documents are free, while some are sold.
GPS, (IS-GPS-200n is the current interface spec, also often called ICD-200)
RTCM SC104, various specifications for differential corrections and NTRIP
NMEA-183, a popular ASCII based protocol to express positional fixes
And of course all vendors have unique protocol specs for their own devices which you can obtain from their web sites.