First off, don't drive the thing anymore untill you getr it fixed, unless you want a cracked head etc ( unless you disconnect both heater hoses under the hood and cap them off, not recommended in winter ).

You'll need to pull the covers under the dash and around the heater core and hoses where you can get your hand in there with some clean rags and feel around untill you can determine where the anti freeze is leaking out from ( when the rag comes back from a spot wet, you've found the culprit )

check carefully arount the heater controll valve and the hoses as these are usually where you'll find a leak.

if it is comming from the heater core, That will be a bit more fun to replace.