You can do three different kinds of tilts: up tilt (utilt), forward tilt (ftilt), and down tilt (dtilt). They are performed by tilting the control stick slightly toward one of those three directions and pressing A. The common mistake is to hit A too soon and do a smash attack or to tilt the control stick too quickly and dash. A tilt can be performed from a standing or a walking position, but not a run or dash (hence dash attack).
The tilt is a very important aspect of marth's game. Learn all the different combinations of attacks leading into a tilt. Tilts are more useful on some characters than others. You must change your game for each character (and not just for deciding when to tilt). If you want to get good quickly, pick sheik
If you want to play the best character ever which can beat any other character if played correctly, pick marth.
I hope that helps. Now go own a fox or falco or sheik with utilt.