You can pick your provider. Most carriers offer data plans for iPads, giving you access to their 3G networks.
If you with to use LTE, then you are tied to the carrier specified by the model iPad you own; either AT&T or Verizon. Look in the small print on the back of your iPad. Model A1430 is AT&T, A1403 is Verizon. LTE currently works in the USA only.
If you have an iPhone already, then it may be easier / cheaper to use this as a Personal Hotspot. You will need to make sure that your iPhone plan allows tethering, but that may be cheaper than paying for an additional plan.