how do I connect a second monitor to iMac 24 with M1 chip

The 24-inch iMac only supports a single external display using a USB-C to DisplayPort cable, or a USB-C to HDMI 2.0 cable.

You can also connect to a VGA display or DVI using the appropriate usb-C to VGA/DVI adapter.

Or get a TB3 dock. They usually have a display connection:

https://www.owcdigital.com/products/thunderbolt-3-dock-14-port