I'm trying to make a bomb item but I'm not sure where to start.

To apply damage to enemies, it may be easiest to use Unity’s physics to obtain enemies within an area. This reqiures your enemies to have rigidbodies/colliders. From here: Clean(est) way to find nearest object of many (C#)

    Collider[] colliders = Physics.OverlapSphere(center, radius);
    float minSqrDistance = Mathf.Infinity;
    for (int i = 0; i < colliders.Length; i++)
    {
         float sqrDistanceToCenter = (center - colliders[i].transform.position).sqrMagnitude;
         if (sqrDistanceToCenter < minSqrDistance)
         {
             minSqrDistance = sqrDistanceToCenter;
             // things within range of bomb here, apply bomb damage to things here
         }
    }

To update that for 2D physics, you’ll just want to Physics2D.OverlapCircleAll with 2D colliders. The link above is a good thread worth reading for pros and cons and different approaches to this problem.

To limit to enemies of a tag, you can add

         if (sqrDistanceToCenter < minSqrDistance && hereBeTheTagCheckLogic)

and you’re good to go. If your enemies use a Health component (or wherever they store their health), get that component and apply damage or kill if within the range.