I found some obvious small blue/purple veins on my thigh skin, what could it be, is it bad ( i am 16) ?

The color of veins: If you are overweight this could be the early signs of stretch marks. If these are just veins and you have not noticed them before then I am at a loss as to what to say. If you have pain, redness or swelling you may wan to see a physician.

Spider Veins: What you describe sounds like spider veins. These can occur during teenage years and there is a strong genetic relationship with them. Trauma can also cause this. The simplest treatment for them would be either a laser or sclerotherapy. You should see a vein specialist for an opinion.