Well .... That depends on where you're from Hilary.
If you're coming from Bangkok, yes, Toronto in December will be cold. If you're coming from Edmonton or Montreal, you'll find Toronto not so bad.
Obviously every December is different, but it is unlikely to snow in Toronto in December (rain is more likely). If it does, it won't last, though the streets might be slippery.
Bring a jacket that protects you from the wind and rain. A pair of light gloves is good if you're carrying anything or don't have pockets (I lived in Toronto one winter and didn't wear gloves once, however ...)
It won't be sandals weather, but you'll be fine in any closed toe shoes, no need for boots.
What you pack really depends on what you'll be doing.
Weather websites will give you the average temperatures and precipitation for December -- you'll be the best judge if whether you find it cold or not so bad for December. about 4 years ago