What Mike said.
The first thing you need to do is find out if your floor is suitable for tile. The big concern is deflection (AKA bounce).
If you put down tile on a floor that is too bouncy you risk cracked grout, loose tiles, and a general mess.
As Mike said, knowing the size and span of your joists is the first step, along with finding out exactly how your sub-floor is constructed. If you have a floor vent, pulling the cover off is often a good way to get a peek at your sub-floor.