Step Up

[Crexis]

Question about this feat. What's the max I can move when I use 'step up' vs an enemy moving on his turn?

And if the enemy decides to attack me, does he gets his attack in first or do I?

[Gary Bush]

The feat allows you to take a guarded step if an adjacent foe takes a guard step. It uses your reaction.

so you move 5'.

Since Step up is a reaction, it is not on your turn, it is on his. The other creature could take a Guarded Step, your reaction to that with a step up, so he can use his standard action at attack you. You don't get an attack by using Step Up.

Step up and strike does allow you to make an attack, which would happen happen before the foe would attack.