1) Style - it's very important to know where you are heading with your design/look, collect as many pictures as possible, pick a couple of styles that you really like, make sure you know what 'final look' you are after;
2) Do some research - look for a supplier online, read a few articles, find out more about the stone you are thinking of using;
3) Get familiar with some of the terminology - honed, filled, unfilled, etc.;
4) Visit a supplier to make sure that you know what the product looks like + ask as many questions as possible (now is the time) and make sure that the stone you choose is the right one for the job/area.
A favourite of mine is Carrara marble, I like to use it in kitchens and bathrooms, for it has a timeless appeal and once treated it is very easy to maintain; it can suit different styles too which is fantastic and marries well with numerous paint colours and styles of cabinetry; it also looks great used as a detail (see here)
Another favourite of mine is travertine, I love the variation of colour and texture plus it looks great both inside and out, the larger tiles look particularly striking outdoors (see link to project below)
more pictures here.