If you don't take one of your flights without notifying the airline in advance, you will be marked as a 'no-show' in their system and the rest of the itinerary will be cancelled. In most cases, this means that the ticket value is forfeited, and it will be difficult to make any claims or requests with the airline regarding the ticket.
If you do not take your outbound flights, your return flights will automatically be cancelled by the airline and you will have to purchase new flights.