What happens if I cancel my flight? If I canceled my flights and want to reserve again at the same schedule, can I still get it?
If you've purchased a non-refundable ticket (traditionally, the cheapest tickets available), you won't be eligible for a refund if you cancel your flight. But, most airlines will reissue the ticket for travel within a year of the original travel date, or will allow you to use the value of the ticket on a future flight on the airline, also within a year of your original departure date.
To cancel, you must contact your airline before your scheduled departure or you risk forfeiting the entire cost of the ticket. You'll still likely have to pay a change fee, which can cost upwards of $100, depending on the airline, so it's not a good idea to cancel your flight unless you have no other option.
If you must cancel your flight due to a serious illness, or the death of an immediate family member or traveling companion, the airlines are usually more lenient with refunds. You may be asked to provide a copy of a death certificate or a doctor's note in order to receive a refund, however.
As to your second question, there is no way to know if the flight you originally booked will still be available, either due to a schedule change or due to a completely booked flight. You should avoid canceling your flight until you're absolutely sure you won't be able to travel.