how to calculate resistors' value?
There are two unknowns: \$R_s\$ and \$R_p\$. I.e. you need two equations.


1st equation:
\$V_g\$ with \$R_g\$ and \$R_s\$ form a Thevenin source with \$V_{Th1} = V_g\$ and \$R_{Th1} = R_g + R_s\$

Transform this Thevenin source into a Norton source: \$R_{N1} = R_{Th1} = R_g + R_s\$.

Include \$R_p\$ to form a new Norton source whose \$R_{N2} = R_{N1} || R_p\$.

Transform it back to a Thevenin source \$R_{Th2} = R_{N2} = R_{N1} || R_p\$.

The resistance of the final Thevenins source is supposed to equal that of   the starting Thevenin source, i.e.

\$R_{Th1} = R_{Th2}\$  i.e.
\$R_g = R_{N1} || R_p = (R_g + R_s) || R_p\$
2nd equation:
You get from \$V_g / V_o = 0.125\$
It depends how \$V_o\$ is defined (I think the problem is not clear about this): either with \$R_L\$ attached or without \$R_L\$ connected. I'm not sure which one is the case (the diagramm actually says with \$R_L\$; but \$R_L\$'s value is not given).
In either case use the voltage divider formula.