Is there any way to tell my iMac to turn on via tasker. If so, would both devices have to be on the same network? Is there a way to do this without being on the same network? Would my computer have to be plugged into the router or can this be done wirelessly? I guess I am trying to use wake on lan. So for instance, when I come home and tap my phone to a NFC sticker, my computer would turn on. Is this possible?
Yes, you are talking about Wake-on-Lan. And as far as I know, the device to be awoken this way must be plugged-in via an ethernet cable (how else? when it's turned off, so is its WiFi).
As for the network: either both devices have to be in the same network, or you have port-forwarding enabled on the router connecting the different networks (otherwise your target cannot be reached).
Now for the remaining part: How to use your Android device to trigger this? You were asking for Tasker -- and yes, that should be possible. There is a Locale Wake On LAN Plugin (and Locale plugins do work with Tasker. I cannot see that Tasker natively supports NFC -- but there's e.g. a Youtube video, so there must be a way -- hm, maybe this Locale NFC Plugin?
So now there should be everything complete, and you can start testing :)