Q1 is a NPN transistor and it works as a low side switch to provide the gnd to the relay and turns it on only when the transistor base is high, it can be any model that can work with the intended relay current, for example 2n2222.
What is the operation voltage of your relay?
Depending on the available supply voltages in your project you should choose a relay that use that voltage too, for example if you have only 5v available then don't buy a 12v relay, on the other hand if you have both 5v and 12v supplies available then buy a 12v relay with uses less current.
Alex