Right after you defeat Ard, the bug maniac for the first time, go to the most northern point of the park. Slightly to the right, you will notice that a piece of the fence is missing. If you go right here there is a secret path going off to the right. You will be able to tell where it is as the trees look different there. The path leads to a hidden area where there is a girl. Make sure you do not have a full pack of pokemon. Fight her chansey and delcatty, and if you win she will give you a Level 1 Treeko.