Don't bother trying. The system has no payback.
A pressurized carbon hydrogen storage tank with a cryogenic cooler on it to keep your hydrogen where it is will probably cost the same as 10 years worth of gasoline.
Electrolysis, the process you'd use to convert water into hydrogen, is VERY power intensive, so you'd pay more money for the energy to create the hydrogen than you would for just burning gasoline. My answer to the post 'What is the deal with the water/gas hybrid kits for your car?' has a rundown of the efficiencies. Ignoring equipment costs, it costs twice as much to run a gallon of gas worth of electrolysis equipment than it does to run a gallon of gas.
Most electric power in the US comes from burning oil or coal, so you'd end up with a carbon footprint larger than you started with, a vehicle you can't fuel up anywhere but your electrolysis machine, a vehicle no service tech can ever work on, a risk of massive fines for running an uncertified high pressure installation, and a total fuel bill that won't pay for itself within the lifetime of your vehicle.
And for the love of god, don't use NaCl(table salt) and water for any sort of electrolysis to get hydrogen and oxygen! It will create Chlorine gas, which is ridiculously reactive, and can kill you in ways that, if I was killed, I'd say "Wow, I just got killed!"