high-resistance continuity detector
Years ago I had to design the electronics for a pH sensor, which have an impedance between 1GHom and 10Gohm, and your problem is the same, how to sense such small signals. The secret to success is to use a very low bias current operational amplifier and isolate it. As a start I suggest to take a look over the circuit note CN-0326, they explain all much better than I the problem and how to solve it, and take a look for their very low bias current OP amps, especially the LTC6268, I used it for my design and works quite well.

Also, there is a book named "Low-Level Measurements Handbook, Precision DC Current, Voltage, and Resistance Measurements" that will help you to understand how to deal with low measurements from the connection point of view (also important here, grounding and stuff).

After that, I suggest you come back and ask again if you have more doubts. Measure low signal is a big topic in electronics and you need to get some background of the key points to make the correct questions and we able to help you properly. And doesn't matter if you don't understand much, that's why we are here.