Who is the first person affected by the love potion in "A Midsummer Night's Dream"?
While Lysander's eyes are the first that are annointed with the love potion, one can say that it affects many people.
Of course, it effects him as he falls in love with Helena. Remember he ran off into the woods with Hermia with the intent to marry her. It also affects Helena because she is confused and thinks he is joking with her. Finally, it affects Hermia because she is hurt that Lysander has seemed to change his mind about her so quickly.
If by "person" you mean a character in the play it is Titania the fairy queen who first comes under the influence of the love potion. Oberon squeezes the juice of the flower into her eyes when she is fast asleep in the forest.
If by "person" you mean a human being it is Lysander who first comes under the influence of the love potion. Puck squeezes the juice of the flower into Lysander's eyes mistaking him for Demetrius into whose eyes he should have actually sueezed the juice of the flower according to Oberon's orders.