The Raspberry Pi is a great little computer but here are the main parts
Linux
If you just starting on it download NOOBS os for it LINK
Also learn a little about command line for linux like (ls, cd, etc)
Raspberry PI
GPIO( General-purpose input/output ) This will let you power devices up from the Raspberry Pi and turn then off and on plus this i mainly done though python
Thats all the basic stuff with it