If theta is in Q2, I'll assume you mean that tan theta = -sqrt(3)/3 since tan theta is negative if theta is in Q2.
Also, since theta is in Q2, sin theta must be > 0.
Imagine a right triangle where the side opposite theta is sqrt(3) and the side adjacent to theta is -3.
Use the pythagorean theorem to find the hypotenuse.
Once you know the hypotenuse, sin theta = sqrt(3) / hypotenuse.