I have some waypoints, and I need to input them into Google Earth.
How can I convert data from GPS to KML file?
You have no need of this conversion if the GPS data is in a GPX format. Google Earth can import this directly.
Typically (speaking very generally) a consumer grade GPS device is going to record GPS waypoints in GPX format, but there are other possibilities. KML may well be one possibility. Typically smartphone based GPS data can be exported in more than one format - GPX would be a common one, and KML another possibility.
There are a whole load of other file formats around.
There are many many pieces of software, many open source or free, or web-based services, for dealing with GPS data (including waypoints, in multiple formats). Many of these will allow export as KML. Google Earth will inport GPX directly, and can export as KML too.
Do a simple web search on "convert gps to kml" for plenty of ideas. Without knowing the format (and content) of your data this question is too broad to answer with anything other than broad answers...
GPS waypoints are just single points with a latitude and longitude recorded (and possibly some additional information).