If this is the Factory Alarm, you should be able to DisArm by turning the Driver or Passenger Door Key Cylinder to Unlock or Unlocking Doors with Factory Remote.
If not there may be a problem with the Security System.
Be sure the Battery is fully charged.Disconnect Positive Battery Cable.Open Driver Door & LEAVE Open.Turn Ignition to ON (run) position & LEAVE On.Reconnect Positive Battery Cable.Alarm may sound for a few seconds.Start vehicle.
If none of the above work, you may have a Aftermarket Alarm & need to find the Valet Switch / Button which is typically located under Driver Dash, could be Red Light, or could be part of the Alarm Antenna.Once the Valet Switch / Button is found, push in & hold for apprx 5 seconds or until the flashing lite stays on or off
Otherwise, you may have to find the Alarm Unit & find the wires that is preventing the vehicle from starting