Simple and cheap way to control baseboard heater from a microcontroller?
Since you're a beginner, I'll give a simple answer: Use a solid state relay (SSR).

For your purpose, they work mostly like a mechanical relay.  However, instead of a coil to control the switch, the switch is controlled by light and your circuit is really switching a LED on and off.

Find what the maximum current of your heater is, then get a solid state relay rated for at least that current, and whatever your line voltage is.  On the microcontroller side, you're really just driving a LED.  The SSR datasheet will tell you how much LED current you need to turn on the device.  Typical values are around 10 mA, but will certainly be way less than the 200 mA your mechanical relay wants.