For $p=23$, $g=2$, $q=11$ you are given the signature $(18,3)$ in the message $m=2$. Construct a signature in the message $m'=3$
Just read the original paper for ElGamal signatures. Especially one of the attacks in section IV. B should help you out. Alternatively, the Wikipedia article about ElGamals signatures also has a section about existential forgeries.

Since this is clearly homework, I'll leave the rest up to you. One last hint: Use q instead of p-1, since you're actually in a smaller multiplicative subgroup.