#1 Go back and add your location to your profile, yes it makes a differance.
#2 Going to need way more details. And a picture would sure be nice.
What's the width, spans, and materials the joist are made of?
What's the spacing of the joist.
What type decking do you have now.
And yes all of this is important. Tile can not just be layed over just any floor dispite what they show on those DIY shows.