Hi!!
I found this simple OpAmp circuit online. I now that it is a low pass filter but i can't find the equations explaining how to pick the values for each component for a given input and a given gain.
Any help would be great! Thanks a lot.

 

DC gain is -R2/R1.

Crossover frequency when reactance of C equals R2, i.e. fc = 1/(2πRC)

See this:
http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php