Pokemon Heart Gold and Soul Silver - Walkthrough - Violet City Pokemon Heart Gold and Soul Silver: Walkthrough - Violet City
In the Borderhouse, you should have received the VS. RECORDER from Ethan/Lyra.

There is a lot to do in this city; I will give you only what you need to progress in the story. More detailed information can be seen in the Locations section of the FAQ.

First, heal up at the Pokemon Center. Then, head up north, across the bridges, and you will see a Pagoda (tower); go inside.
This walkthrough was written by MKaykitkats, used with permission.