There is no charge to your iPhone's wireless bill to use an Apple Watch. It connects to your phone via bluetooth, and there's nothing your carrier needs to know about it.
Any data the watch needs will be pulled from the phone, so updating weather, calendars, etc., may use your phone's data plan to update but that's it. It also doesn't continuously update -- there are limitations on how often it can pull data. Haven't had any issues with excessive data usage since I got the watch, it's all very minimal so far
When you get your watch, make sure to head over to the Apple Watch forums here on iMore and post a picture of it in the "Show off..." thread!