Transfer function of a basic RC circuit
At the risk of doing your homework for you,

Start with the voltage divider rule

$$\frac{V_o}{V_i}=\frac{Z_C}{R+Z_C+Z_C}$$

where \$Z_C\$ is the impedance associated with a capacitor with value C.

Now substitute

$$\frac{V_o}{V_i}=\frac{1/sC}{R+2/sC}$$

Now multiply by \$\frac{sC}{sC}\$

$$\frac{V_o}{V_i}=\frac{1}{sRC+2}$$

Now divide both the numerator and denominator by \$RC\$ to isolate \$s\$.

$$\frac{V_o}{V_i}=\frac{(RC)^{-1}}{s+2(RC)^{-1}}$$