Can I use only a part of my flight ticket?
In general you will have to use all flight segments in your ticket in the order you booked them. Of course you are not forced 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, the remaining (return) part of the ticket will be lost.
The same 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.