I'm presuming that you've got a non-refundable fare otherwise this is a fairly pointless question - just contact the airline, cancel your ticket, and get your money back.
If it's not a refundable fare, the best option is to NOT cancel it - yet.
There are a number of things that can happen between now and when the flight is scheduled that can give you cause go to the airline and ask for a full refund, even for tickets that don't allow it.
The first of these is a "schedule change", where the airlines sometimes rearrange their schedules and thus change the times of flights. If the change is sufficiently large (eg, more than about 2 hours) the you can normally contact the airline and ask for them to refund the ticket.
The second is "waivers", which airlines sometime publish around times of bad weather/strikes/etc. These waivers are basically the airline giving passengers the option to cancel/change their plans in order to reduce the number of services they have to fly. Generally these waivers are short (often a day or two, occasionally as much as a week) and will only cover one or a small number of airlines. Keep an eye on the airlines website in the days before the flight to see if they have such a waiver covering your flight.
The odds of either of these happening are low - but they do happen so it's worth checking!
If neither of those happen, then it's generally good form to contact the airline a day or so before your flight and cancel the ticket. It's possible that by doing this you'll end up with a "credit" for future travel, as even most non-refundable tickets can be canceled - but there'll normally be a fee involved. If the value of the ticket is less than the fee you'll get nothing. If it's more, you'll get the difference.
If you know that you're not going to get a credit back, then there's no "need" to cancel the ticket - the airline won't "black mark" you or anything like that - but it's still good practice to do it anyway. If nothing else, think of the person who needed that seat but wasn't able to get it because the flight was full - by canceling your ticket a day or two before, you give them the option of getting onto the flight!