I booked a flight with United Airline and need to cancel. Will I still have to pay for my ticket? How do I cancel reservation?

Call your travel provider and explain the situation to them.
There may well be a penalty depending on how much notice you give before the actual flight date.

You call them... if you got a cheap non-refundible ticket... you might not get anything back at all.

This is a write-up about their refund policy.
Trip Cancellation & United Airlines Policy | USA Today (http://traveltips.usatoday.com/trip-cancellation-united-airlines-policy-108137.html)

But all depends on the stipulations which you would have had to have reviewed when you purchased the specific tickets.
You can probably go back to the same site you used to purchase your tickets (as if you were purchasing new tickets) and review the cancellation and refund policies, if you have no other written requirements in your possession.

Why would you post here and not just ask where you purchased the ticket or UA directly? Without knowing the conditions under which you purchased the ticket, we really can't answer.