I'm a newbie to CC as well as to cake decorating (only 2 months).
I want to try to make ganache, I found a chocolate bar at my local Giant Tiger store which is Belgian Extra Dark Chocolate with 77% cocoa solids. Is this OK to use or does it have to be baking chocolate?
That'll be just fine. It will be a dark ganache using that chocolate, but I think that's the best kind of ganache!
By baking chocolate do you mean unsweetened? I wouldn't use unsweetened for ganache; it'd be nothing but bitter.
You can use it, but it will not be very sweet at all, so keep that in mind which kind of cake you are using it for and your client's preference.