HTC One comes with a Micro SIM card slot support. There are two edition available in this. Once is single sim and other one is dual sim. It is a bit tricky to insert SIM card in HTC One. So you can follow my guide to get safe methods.

First arrange a Micro SIM. If you have a full size sim you have to use a Sim cutter to reduce the size. You can checkout a tutorial video here - How to cut Sim Card

Take a pin and insert it slowly into the tiny hole next to SIM Slot. Gently push it inside. The SIM Tray will pop out. Remove the tray.

HTC One Mini SIM

Place your card properly on that. And then insert it in the phone once again. That’s all. The sim is no connected. It is necessary that you use a pin and push it gently. If you try hard you can damage the tray.