Heu!!! I don't understand.
Is it means that from my transistor, do I have to connect only the GDN of arduino card and not to -12V ?
The term "ground" is a reference term; voltages are referenced from a "ground" level; if that ground level is 0 volts, you can have voltages above or below this level. If you had two 12 volt batteries connected in series, and called the center tap "ground", you would have one end at +12 volts, and the other end would be -12 volts.
"Ground" is a reference level, and considered "0 volts" for reference, with other voltages in the circuit referenced to this.
Since you don't know this, I have to wonder if what you have marked on your schematic as -12 volts is actually "ground" - and you are calling it -12 volts because you are hooking up to the negative (-) terminal of the battery or power supply?
If that's the case, don't call it -12 volts (that means something different) - it is the "ground" (0 volts) reference. And yes, you have to connect the grounds together (unless you really are using a dual-ended supply - but I doubt that because you didn't know what I was talking about when I mentioned it).
Yes, It is a automotive relay, the only one I found with a 40A.
That's 40 amps at 12VDC; which is a world of difference than household current. You need to find a relay with contact ratings of your amperage for the voltage it will be used at (230 VAC); actually, if your amperage is 40 amps, you want a higher rating (never use parts at their ratings; those are max values, and you never want to do this - instead, spec for parts for 20 percent or so over what you need; so for this, if you were expecting a 40 amp load, select a size of 50-55 amps or greater).
You're probably not going to need a 40 amp contact relay for 230 VAC; I doubt a toaster will pull that (the rating of the plug/outlet is probably less). You need to find out what the toaster's maximum amperage rating is, then select a relay with contacts rated for that value at 230 VAC.