Firstly, congratulations on your phone. Once you start using wireless charging you'll be hooked.
I have Samsung's own EP-PG950 wireless charger that was marketed alongside last year's S8 and S8+. That supports two modes of charging:
- Normal wireless charging, at 5V and 2A input (i.e. 10W input) and 5V and 1A output (i.e. 5W output)
- Fast wireless charging, at 9V and 1.67A input (i.e. 15W input) and 9V and 1A output (i.e. 9W output)
Samsung's flagships support both normal and fast wireless charging.
Now, the Qi standard itself specifies a minimum of 5W output (i.e. the normal wireless charging listed above), so any Qi charger will support your phone.
If you stick to brand name Qi chargers (e.g. Samsung itself, or Belkin, Anker, and other such big names in the mobile accessory space), you will be fine.