How to find the transfer function of this first order active filter?
You need nothing than the equations of feedback theory for opamps.

The general form for the closed-loop gain is 

Acl=Hf * Aol/(1+Hr * Aol)

which simplifies for infinite openloop gain Aol to

Acl=Hf/Hr.

Here, the function Hf is the forward damping and Hr is the feedback function. Both are defined as follows:

Hf=Vd/Vin for Vout=0 (grounded) with Vd=diff. voltage at the opamp input nodes.

Hr=Vd/Vout for Vin=0 

This way, the problem is reduced to simple voltage dividers.

Alternative(Edit): Perhaps the following method is easier to understand:

For an ideal opamp we have Vd=Vp-Vn=0 >>>>Vp=Vn

1) It easy to find Vn=f(Vout)

2) Vp=f(Vin, Vout) needs application of the superposition rule.

From both equations it shouldn`t be a problem to isolate Acl=Vout/Vin