Your greenhouse should warm up nicely during the day when the sun is shining.  The easiest way to heat it would be with a small electric or propane heater set in the center aisle.  If you cannot monitor the temperature in person, you will find it easier to have a small heater designed for heating greenhouses that have thermostats on them so they will shut off and come back on automatically.

Other methods of heating a greenhouse are with metal cans painted black and filled with water, but they take up a lot of the room inside a greenhouse.  They will absorb solar heat during the day and emit it slowly during the night. 

Your greenhouse will have many different temperature zones inside it, heated or not.  You should set min-max thermometers in every nook and cranny so you can know where the colder zones are.  Of course, it will be colder nearest the glazing, so pull your plants toward the center away from the glazing to protect from cold zones.

Growing 'cold' crops in your greenhouse means you don't have to heat it up like a suana, but start your seedlings on a heat mat to give them a good head start at growth.  Once you have transplanted them, they should be fine without the mat.

And make sure you keep an air circulation fan going.  Plants need air moving around their leaves to survive.

Polycarbonate is the best material you can use.
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