There should be 2 or 4 bolts that hold the clamp for the handlebars and probably you just need to tighten those to prevent the bars from rotating inside the clamp. Usually these are metric Allen bolts, but I have no idea about your bike because those sometimes don't use standard components.

Use the gears to keep your feet turning as close to the same speed as possible when you ride even if it means you slow down or speed up. This helps you ride more efficiently because your legs have an optimal speed when you pedal. Once you find your preferred rate of pedaling, change the gears to keep that pace.

I am not sure what you mean you have to stop for a bit. If you mean that you have to stop to change gears, there is something adjusted incorrectly. If you mean you have to stop due to fatigue, don't worry about it, stop as necessary, but not more than 2 or 3 minutes. Over the next few weeks, you'll get stronger and you won't have to stop as often.

Canadian Tire...I dunno if I'd trust them to do tune ups because from what I can find they don't have full service bike shops and maybe not even professional mechanics. Maybe MEC can do it if it's like REI in the US. There may be some bike shops in your area too that may or may not do tune up for you. Many bike shops won't touch department store bikes for various reasons.