Start from the basic conductivity equation (in 1D). This is:

Steady state means:

giving now:

Because this is only depending on x, we can write ordinary derivatives instead of partials and the solution is:

Applying now the boundary condition, you will have a simple result. Can you do this and interpret physically what it is? After that how would you calculate this value if f(x) was given? Expanding it to more dimensions will be fairly straightforward once you solve this 1D case.