123jeffrey,
catch a Cottonee and trade it for the Petilil in Nacrene City. The Petill you receive can evolve only after fulfiling these three conditions;
1. Cure your entire team of status problems using
Aromatherapy. All status problems must be picked up in a single battle. This must be done once.
2.
Leech Seed a Fire/Poison/Flying/Bug/Ice type of Petilil's level or higher and reduce their health completely, without switching, using any items, or any offensive attacks of your own. This must be done once.
3. As a Petilil, she
can never have the highest level in your party (joint-highest is OK). If at any time she becomes the highest, you must level up all others in your party until she becomes the lowest levelled member, before she can be used again.
(These probably sound harder than it is, I didn't want to make it impossible for you
)
Once Petilil evolves, you can do whatever you like with Lilligant's level and moves (if you wanted to get rid of Aromatherapy and/or Leech Seed, for example
)