Probably more than you think as it can seem really daunting at first.
Make sure you know if it's wheat and dairy generally or gluten and lactose as the latter can be in things you wouldn't necessarily realise.
I've done it and managed and it can make you feel a lot better and you may be able to introduce things to your system gradually at a later date.
Make sure you eat as balanced a diet as possible.
Loads of cookbooks available...
http://www.amazon.co.uk/Wheat-Dairy-Free-Cookb ook/dp/009188893X
Most supermarkets also do special ranges.
You can swap things like pasta for rice noodles and corn pasta, do loads of things with potatoes and veggies.
You can get different types of bread free from wheat and wheat free flour and dairy free spread so you can make your own stuff, maybe even get a breadmaker.
You can also use lactose free or soya milk.
Sway icecream for sorbet.