How do I get cellular data on my new iPad?

Do you have a WiFi Only model of iPad, or WiFi+Cellular? Only the WiFi+Cellular models can be directly used with Cellular networks.

Be aware that the majority of Phone SIMs are not correctly provisioned for most tablet computers - and most will offer no service at all with an iPad. In most cases, you will require a DataSIM that is explicitly intended for tablet computers and Cellular Routers (also known as MiFi).

If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

Whilst a WiFi Only iPad cannot be transformed into a WiFi+Cellular model, you can use the hotspot feature of a mobile phone - or a Cellular Broadband Router (often known as a MiFi Router) - to access Cellular networks.

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

if it's the cellular model try reading Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

if it's the wifi only model then you can't