Hi, i'm seventeen years old and i have no arch in my left foot, it's also difficult to walk straight on that leg. what's the issue?

Flat feet?: It sounds like you could have flat feet. You either need to see a podiatrist that will do a foot scan and create special foot insert. This will help to keep your feet aligned and reduce the amount of pain from inflammation that occurs. You could also try some cheap inserts from Dr Scholls Doc Rogu

Straight away....: It is not typical that "no arch" is evidenced in one foot only. I'm also not sure just what "walking straight" means. These issues, as well other potential associated conditions at hand, should be easy to identify and resolve. I suggest a thorough biomechanical and physical examination of your feet and legs, as the perfect starting point.

Oftentimes: When one has a unilateral flatfoot this is indicative of a limb length discrepancy. The not walking straight part if that means your foot turns outward (abduction) that is a compensatory mechanism the foot is doing. The limb length discrepancy if there may be coming as high as your back (scoliosis) or from a pelvic shift or from the lower extremity.