What kind of relay would I need? Sorry but I know only very very basic
electronics.
The relay has two ratings coil voltage and contact amperage/voltage.
The coil voltage has to be compatible with your control circuit
(thermostat side). The contact rating has to switch the current of
the heater (or higher) at some given voltage rating.
At minimum you need a single pole single throw relay normally open
(that is, it will call for heat when the coil is energized). Many
thermostats are designed to run on 24 Volts AC (coil voltage your
relay would need to be, along with a transformer to power it)
The 24 VAC coil rating is used in the states because it doesn't
require heavy wire with a 600 volt insulation rating the way
electrical code does. There will usually be a small 10 Volt-Ampere,
24 VAC transformer located on the heater, as a rule, to step down the
voltage for the control circuit (thermostat).
A 24VAC mechanical thermostat probably wouldn't survive switching
120/240 VAC for very long (if at all). A 24 VAC electronic thermostat
would die instantly with 120 or 240 on it.
There are heating thermostats that will run on 120/240 directly and
would need no relay (just wire the contacts in series with the heater)
You can find them in hardware stores for baseboard electric heating.
They are rather primitive, and use a bimetallic switch and don't
usually allow one to adjust the hysteresis ("dead band" differential
between turning on and then turning off again - a degree or two as a
rule - otherwise the thermostat tends to turn on and off frequently
and that kills the contacts)
Some low voltage thermostats (but not all) will allow one to adjust
the hysteresis.
If you need tight control use a solid state relay. They switch as the
voltage of the ac crosses zero and don't have contacts that will die
the way an electromechanical relay will - you can set the hysteresis
to zero - turns off and on at the same temperature (or damn close)
An electrician could tell you what you need to do . . . if that's too
confusing.
For good tight control a "proportioning" controller is used. It
switches on and off regularly (like every 5-30 seconds) and just stays
on longer to bring the temperature up. They tend to cost more, and
the contacts won't last forever if asked to switch a heavy current.
Some folks call them "proportional" controllers, but a true
proportional control modulates the power by switching like a lamp
dimmer or varying the voltage from 0-100%.