Lewis, you cannot make a sparkplug "spark" with just a 12V battery
You need about 10000 - 30000 Volts to get a spark to jump the electrodes.
How do you get that many volts from a battery? With an ignition coil just like the one car petrol engines have.
So, in your diagram you need to wire the + primary terminal on the ignition coil to the switch and the negative coil terminal to the negative battery post.
The centre of the coil has the spark plug lead attached (its a special wire - do not use anything else, it MUST be spark plug lead :!: )
The other end of this wire goes to a spark plug cap which fits over the ceramic plug insulator, the metal thread of the spark plug base connects to the negative battery terminal.
You might draw a nice circuit again and I will tell you if you got it right.
Perhaps you best get the coil and lead parts from a car scap yard. Your pushbutton switch must be able to handle 3 Amps or so.
Now, when you got all wired up, STOP a minute and think what will happen to you if you touch 30000 Volts
Yes, you will be thrown right across the room and hurt yourself. So DO NOT TOUCH the spark plug lead,the plug or the ignition coil.
Now, what happens when you push your switch is nothing
but, as soon as you let your finger off there would be a spark at the plug. The spark occurs when the switch opens after it was closed for a short time. Do not close it for too long or your battery will go flat.
The other advise you got about a capacitor is a bit misleading, if you charge it from the 12V battery nothing will happen. But, if you charge it from 100's of volts you see fireworks if you touch it. DO NOT try to charge capacitors from the mains voltage - it will kill you.
Klaus