1) How to create a guild

At the moment, the most you can create is a concept. And all that requires is a thought or an idea and finding others with similar thoughts or ideas. The details of how we construct a "guild" in the game is not known yet nor is it really important to know at this stage.

FWIW, there's supposed to be a website update that will allow the community to create personas (ie characters) and associate them with organizations/domains/etc. Once that's available, your concept might be a bit more concrete. In the interim, you can also just create a forum post about your guild. That's as far as it goes for the moment though.

2) Is it possible to buy a building for your guild with exposition points

Yes. Absolutely. Provided you have a high enough pledge package to get into exposition and enough exposition currency to purchase one. The Expo item store is supposed to contain a lot of stuff to help with preparing the world for launch. You'll also need to have a location - either within a settlement or available land within a county.

3) How do you recruit

All recruiting happens out of game for now and there are no binding contracts (for in-game use). So in short, however you want. It might also be helpful to look around the various kingdoms and join one so you can get to know people who might be interested in your guild.