If the flight is cancelled by the air carrier, you are entitled to a full refund to your original form of payment.

If the flight operates and you cancel, then you are only entitled to what the fare rules of your ticket provide. In addition, as you have booked through a third-party, you are subject to any rules of the third-party. Often, these will all include either rebooking to a future date or a credit which you may use towards the purchase of a future ticket. But, no way to provide you a specific answer without reference to the specific rules you agreed to at the time of your purchase.

Although it may no longer be relevant, if you skip any segment of a ticket, the remaining segments of the ticket will be cancelled. The ticket will then retain whatever value it retains under its fare rules. That is most often, but not always, zero. In addition, the rules you agreed to with your third party vendor may apply as well.