Bicycle Tutor

I recently bought an Upland Vengeance, after using my old bike for over 4 years.
One issue I had though, was that when I started pedaling, the bolt on the left pedal would become loose and loose until it simply fell off. It was a bolt with a disc shape at one end, and then a threaded part. I'd really love to ride my bike without stopping every five minutes to tighten it. I plan to take my bike down to a local shop, but I was wondering if there was a very simple DIY remedy to this. I don't have the bolt anymore, it rolled away after falling off. I'm sorry if this topic has already been made.
Also: My bike can change speeds, but it goes by a system of dashes going from an L to an H up and down. I'm wondering if the dashes all just represent numbers from 1-whatever or something else. Any help would be greatly appreciated.

I'm not sure which bolt you mean, and I couldn't find any photos of the Upland Vengeance. If you could upload a photo or two of your pedals (with and without bolt) it would help. If the bolt is missing you'll likely have to find a replacement.
As for you question on gears, check out this video, it may help!