What must Harry do to get on the platform?

This question is likely referencing the Harry Potter series. To get onto Platform 9 3/4 at King's Cross Station, Harry needs to:

* Run straight at the barrier between platforms 9 and 10. It's not a physical platform, but a magical one that only becomes visible when someone runs through the barrier with the right intention.

* Believe it's there. The platform only appears for those who truly believe in its existence.