You need a 12volt Charger/Power Adapter that is compatable with your Laptops input voltage (e.g. 19v). Some have adjustable voltage settings.And you need a packet of 'convertable DC output plugs' to suit your laptop. Some charger/adapters come with them.
Similar to wots here.....
www.computeronline.com.au/products.php?C_ID=26&S_ID=421&PROD=24845 But shop around by all means as this is just an example of what you will need.
I've been using one for years to run my laptop from the ships batterys so as to be able to run my charting system, be connected to the internet via a USB modem connected to my tricky antenna and watch TV through a USB Digital TV Tuner. Too easy.