Calculate the position difference between the transforms of target A and target B.

Transform targetA, targetB;

Vector3 posDifferenceFromAtoB = targetB.position - targetA.position;

By normalizing the posDifference you may get the direction vector from A to B.

If you are not familiar with Unity API, check here :

https://docs.unity3d.com/Manual/index.html
If you are not familiar with some essential mathmatics for (3D) game development, maybe you'd better study basic Linear Algebra first, like this one :

https://www.youtube.com/watch?v=fNk_zzaMoSs