There's a whole, whole lot than can go into it, but a great starting place is a ratio of 17.21:1 water to coffee. That is, if you have 500 g of water, you should use 29 g of ground coffee. What the ratio really does is specify how concentrated the coffee should be. Once you've set your ratio, there are many other variables to play with to make the coffee taste wonderful including grind size and consistency, bloom time, brew time, filter type, pour method, etc.
I wish I could post hyperlinks to all the individual videos of different french press techniques, but you can find them all at the wonderful site http://brewmethods.com Specifically, please check out:
Lastly, and please tell me if this isn't kosher, but I've made an iPhone app that will handle the coffee and water weight conversions automatically, as well as timing the brew and detailing a couple guidelines for different brew methods. It's available on the app store as Brew Control.