Hi, i have been given such pdf
f(x,y) = k/y, 0<x<y, 0<y<1
f(x,y) = 0 elsewhere
How should i construct in order to find k?
Have you applied this advice here? If so, it should be clear that you require the value of k such that $\displaystyle \int_{x=0}^{x=1} \int_{y=x}^{y=1} \frac{k}{y} \, dy \, dx = 1$.
But if you reverse the order of integration the calculation is much simpler ....