Input resistance of a transistor circuit
You need to follow a two-step process to solve this. First you find the bias point, and calculate the value of r\$\pi\$. For that, you use the Thevenin equivalent and Vt. 

Once you have r\$\pi\$, you move to small-signal thinking, and in small-signal terms, the non-input side of R1, R2 and r\$\pi\$ are all connected to ground, so they are all in parallel. 

Here's a nice MIT EdX open courseware lecture you might find useful.