Cellular connectivity for an Apple Watch must be provided by your cellular phone network as an extra function on the service plan for your iPhone. The watch then works as an extension and accessory to the iPhone, and shares the same phone number. Sometimes the point of sale will be able to hook you up with the extra plan, sometimes you must do it yourself.
Working out your options should be simple. First, your iPhone service plan needs to be a pay-monthly contract, because very few providers accept Apple Watch (cellular functions) on pre-paid plans. Then check your cellular provider is listed as providing service for Apple Watch
Apple Watch - Carriers - Apple
If that all looks good, you can go ahead and plan to buy your Apple Watch. The choices are usually to buy from your cellular phone provider, or direct from Apple. In either route you can probably do either in-store purchase or by mail and have it delivered. The one to avoid if possible is purchasing from a third party reseller other than your cellular provider.
If you buy from your cellular service they will probably help you with setup and activation. If you buy in the Apple Store you also have experts on hand to setup before you leave. You can actually setup the new Apple Watch as soon as it is in your hands, skip over doing the cellular part if the plan is not yet arranged and get it working with your iPhone. Then when you get the extra service enabled by your network just do the additional cellular setup. Of course if you buy through your cellular provider you should be able to do all the setup in one session.
Finally, you can always come back here with any questions.