You drive the relays with a voltage signal, which can be either HIGH or LOW. Different relays have different requirements. Some relays need to get 3.3V to operate, while others need 5V. Since the Spark Core operates on 3.3V, you’ll need to find a relay which is compatible with that, or you’ll have to figure out a way to get 5V to it (shouldn’t be that hard if you can’t find a 3.3V).
The amperage in relation with the Spark doesn’t matter, since the current will only run through the relay. The relay will have to be able to handle the current that is necessary for your heater. The only thing your Spark has to do is supply a high enough voltage to switch your relay.
Depending on what voltage characteristics said arduino shield has, you may, or may not, be able to use it with the Spark directly. As far as connecting the two goes, you could use some wires (rather messy), or you could use the Shield Shield, which is basically an arduino shield adapter for the Spark Core.
To recap; the two most important things to watch out for are current ratings of the relay, and the required voltage to switch the relay. The current would be the most important, since it could cause hazardous situations if it’s not cabable of handling those currents.