Hi I have just purchased an ipad pro 2018 and cant find how I use my SIM card

The SIM card on the cellular-capable iPad Pro is on the right side, near the bottom.

Remove or switch the SIM card in your iPhone or iPad - Apple Support

If you have no SIM slot, then you do not have a cellular-capable iPad, but rather have a Wi-Fi model.

Are you sure you have a cellular model?

Take a look at this:

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

Remove or switch the SIM card in your iPhone or iPad - Apple Support

Good luck!