Relay driving transistor will depend on your relay's current requirement .The 555 is designed for high current output that can sink or source maximum of 200mA. If your relay is below this value you may directly connect it.Also the time has an ability to operate from a wide range of supply voltages.When used with appropriate supply voltage, the output is compatible with CMOS,DTL & TTL.
You must be careful when connecting an inductive load such a relay to the output of 555 or any other solid-state device. When the current through an inductive load is interrupted, the collapsing magnetic field generates a high reverse emf (transient voltage) that can damage the device. The solution to this problem is to connect a diode accross the relay coil so that it conducts and absorbs the transient.The diode must be connected so that it is reverse biased in normal operation.Hope you understand.