If there are open housing plots available on your server, then you just buy them directly from the game and not from other players. (This is the only way SE intended for housing to be bought.) For an FC house, your FC needs to be at least rank 6. For a personal house, you need to be at least level 50 and a Second Lieutenant in your Grand Company. The plot itself will cost several million, and then you'll still need to pay for putting a house on it. (You can't do anything else with the plot --not even outdoor furnishings-- until you put a house on it.)

If your server has run out of housing plots, players have come up with workarounds for that, one of which is the buying of an FC that you've heard about. As others have mentioned, there are problems with that, the biggest of which is that your chances of actually finding a reputable FC willing to sell you their Company (and with it their house) are pretty slim.

The other workaround is to pay off someone to abandon their house so that it goes back on the market and you can buy it from there. This tends to be an expensive workaround, since you'll probably need several million gil just to pay them for abandoning it, and then you need another several million to actually buy it yourself. (You also have to make sure you meet the other requirements to be able to purchase a house.) Also, since relinquishing a house wasn't intended to be a tradeable action, you have to just trust that when you give them the gil, they'll follow through on the bargain and relinquish the plot like they said they would.

This wiki has a listing of the starting prices for housing plots (assuming there are any available to buy).