Change Macbook Pro keyboard F keys as default

I need to switch to mirrored display using the command key + F1, but I have to use the Fn key to change the laptop key to F keys, then press the command key, and it doesn't work. I have to connect to an external keyboard which has F1 key without using Fn key to make it work. Is there anyway to lock the F keys so I use them without press the Fn key?