A cadence by its musical term, or better yet, it's purpose is to end what you are playing. In a traditional sense, the last chord of a musical section is the fifth chord or the root chord. I'm not saying you must end with one of these chords, but it is one of the most useful and common ends to this.

You'll definitely go through trial and error, but generally listen for what works for you. No theory can tell you how to right your song, but it can give you ideas and possibly things to better enhance your songs quality, or even things you may not of though of.

Yeah you can repeat a cadence in a song many times, the one you want to save for the end is the perfect cadence V-I, when both chords have the root in the bass and the highest note in both chords are the root of that chord.

Example:

In C major, a chord sequence like G (323003) to C (x3201x).

The part where I was saying not to do so more than once is a song is the stop one section on a weak beat, and change to a contrasting part on the following strong beat.