How do I get 240 volts

I doubt if you can get 240volts from your alternator, as they are regulated to supply a maximum of around 14v to recharge the engine battery and run the lights, radio, heater etc.
So......you have two options, either use an inverter to 'convert' the 12 volts from your engine or leisure battery to 240v, or run a seperate 240v supply from outside and then connect to an external supply as is normal on a campsite, or at home.
It really depends what you are wanting to run at 240v, if it is just to recharge your toothbrush, or run a laptop, then an inverter is an inexpensive way to do it.
If you want TV, X-box, heating, lighting, fridge etc, then you are going to have to run a proper hook-up for an external connection point. If you are not sure how to do it, then get some help, or find a competent electrician to do it for you.

As you are aware car systems are 12volts. Therefore the only way you can get 240volts is to use an invertor be it hardwired by a specialist or an off the shelf bought item. Either way it is a very inefficent way of getting 240volts. If you are using high powered tools on a regular basis you need to buy a generator as this will in the long run cost you less money due to the fuel savings you will make.

KS

or a small generator but depends when and where your using it dont want to be annoying local campers