Here is how you proceed :
Reduce the system to either unity feedback system or a Single block representing the closed loop transfer function. In your case, reducing to a single block should be easy.
For a unity feedback system , as shown
Then from the diagram
E(s) = R(s) - Y(s) = R(s) - G(s)E(S)
E(s) = R(s) / [ 1 + G(s)]
If you reduce it to a single block, Then
E(s) = R(s) - R(s)G(s) = R(s)[ 1 - G(s)]
If you have non unity feedback system, then
G'(s) = G(s) / [ 1 + G(s)H(s) - G(s)]
will reduce it to the form in figure.
For steady state error, you need to specify the input. 3 inputs are used :
- Step with Laplace transform 1/s
- Ramp with Laplace transform 1/s^2
- Parabolic with Laplace transform 2/s^3
Substitute the transform of input into the equation for your error obtained in step 2.
Use the final value theorem acc to which
E(infinity) = lim s-> 0 [ s.E(s)]