How can you target an ally to perform a heal on them instead of yourself?
Being able to heal others depends on what spell you're using:
You can't heal others with Healing, Fast Healing, or Close Wounds: they're self-cast only.
You can get Healing Hands and Heal Other, which let you heal most targets (including followers). They work just like offensive spells: point and shoot.
You can also get Grand Healing, which is an area-of-effect heal. This heals everyone in the area of effect, though: including enemies. This is not targeted: it heals everyone within range of the caster.