Ground Connection for circuit.
Why do you think you need a ground connection? In most cases ground is just a reference, so we could call the negative connection ( the one with the "-") of your battery "ground". There is no need to connect it to anything else. Your circuit will operate on it's own.

For some circuits, ground is an actual connection to the ground, meaning a metal pin you stick in the earth. But this is mainly for the mains circuit (110 or 240 V AC) in your house or for some radio transmitters/receivers. Don't worry about these yet, when such a connection is needed, you will know.