That depends.
All iPad versions come in two models. One model supports both WiFI and Cellular access (with an appropriate plan from a cellular provider). The other model supports WiFi only.
If your iPad is the WiFi only model then you have no choice (other than buy a new cellular-capable iPad).
If your iPad is WiFi and Cellular model then you can activate a cellular plan for the iPad and get online. Generally this is best done through your existing provider (assuming you already have cellular phone service) since they usually offer discounted plans for linked accounts.
The easiest way to tell if your iPad supports cellular connections is to check Settings. In the first block of settings (alongside Airplane Mode, WiFi and Bluetooth) there will be a 'Cellular' option if your iPad supports it. If there is no Cellular option, then your iPad does not support cellular connections.