My truck is leaking green oily liquid onto my passenger side floorboard I've read that it might by coming from my heater core How world I fix this?

Yes, the leak definitely sounds like a leaking heater core. Sometimes they are easy to replace, and sometimes they are difficult. You can try some radiator stop leak in the radiator but most of the time they have to be replaced. If you do it yourself, get a good repair manual from an auto parts store. It will have the instructions on how to replace it.