Touch = Melee range for all intents and porpoises. It does not tell you about what the power's attack roll/saving throw/action is.
The text of the Lay On Hands ability states:
As an action, you can touch a creature and draw
power from the pool to restore a number of hit points
to that creature, up to the maximum amount remaining
in your pool.
From Range (Players Basic p79)
The target of a spell must be within the spell’s range.
For a spell like magic missile, the target is a creature.
For a spell like fireball, the target is the point in space
where the ball of fire erupts.
Most spells have ranges expressed in feet. Some
spells can target only a creature (including you) that you
touch. Other spells, such as the shield spell, affect only
you. These spells have a range of self.
Basically, if you are adjacent (on a square grid this is in an adjacent square, in a gridless style, this is 5' or less away (or in melee range without reach if that's what you're tracking).
Lay on Hands doesn't require an attack roll, it just happens. There is not a specifier that the creature is willing, again, if you can touch them it happens. Your DM may want to include a saving throw or make it a melee attack, but this is would be a house rule, not a RAW interpretation. The RAW here is that if you're in range, and there are no qualifiers, the effect happens. If you want to add a house rule, take a look at the Light cantrip for a solid implementation.
Also, it's worth asking yourself why you want to use it on an enemy? It has no negative effect on them, and would mean you are restoring them HP.