I think the easiest way to get started is mapping. This is a very cool map editor for Q1 and Q2:
https://github.com/kduske/TrenchBroom
There's plenty of good content here:
http://forum.tastyspleen.net/quake/index.php?topic=21859.0
http://maps.rcmd.org/tutorials/
Map compiling and texture tools are included in the engine code:
https://github.com/klaussilveira/qengine/tree/master/src/too...
The Quake community is awesome: https://quakewiki.org/wiki/Mapping_tutorials