Can I use only a part of my flight ticket?
In general you will have to use all flight segments in your ticket. Of course nobody can or will force you to use the return portion of your ticket if you decide not to go home. But you are not allowed only to use the return flight and skip the departure flight - in cases where you do not use the departure ticket all of the ticket will be lost.
The same thing applies to tickets with stop overs - you must fly all portions of the ticket and cannot start the journey in the middle of the route. For instance if you have a ticket New York - Los Angeles - Bangkok you are not allowed to start the journey in Los Angeles but you have to start in New York.
If you have a 'KILROY ticket' for youth and students you should contact KILROY for exact rules as these tickets most often can be changed to match your new desired itinerary (against a fee).