Bicycle Tutor

I just purchased a used bike a couple of month ago, and now the left crank is getting loose and i have to tighten it. Every ride the crank is only attached through a screw to the bracket. Do i need to insert a ring with that screw? or maybe there is some trick that i am missing?

The crank is just held on with a bolt. There is sometimes a washer under the bolt. But sometimes the bolt flares at the bottom of the head with a 'built-in' washer.
If this keeps loosening up it probably means either:
- You're not getting it tight enough
- The crank arm is damaged
Riding with it loose will eventually damage the arm to the point where it won't stay tight at all and you'll need to replace it. I'd try riding into a shop and ask them to tighten it properly for you. That's the kind of thing they'd normally do for no charge.

i just got a wrench and tightened it quite well
plus i added a ring under the screw to help
if that fails i'll go to a bike shop and replace that crank

An old trick is to heat up the crank arm at the "square" end (in hot water) before fitting it. This causes the "hole" to expand and allows the crank to go further onto the axle.
This did cure one of mine a good few years back but I never tried to remove it after. Make sure that the square hole and axle are clean with no burs etc. If you don't fancy the heating method then try a small amount of grease.