Which is the air resistance formula?

I'm trying to solve this ecuation:
Y_1=(-m)/2k((T-mg-kv^2)/(T-mg))
I'm having some truble at finding a correct formula to find the air resistance. In my research through Internet I found loads of different formulas but I dont exactly know which is the correct one. In an other discussion on this forum I found this formula:
F_D=(C_D ρAv^2)/2
However, as I just mentioned I don't have it very clear.
Thankfully,
Jacob