Gain a knowledge of key signatures and become comfortable spelling and identifying the I, IV and V triads for each major key (when comfortable with all the major keys, focus your learning on the minor keys and triads). There are a number of sites online that offer information on key signatures and their triads.
Cadences in music occur when a harmonic progression closes a phrase, section or piece. Grab a simple piece of piano sheet music that contains a left-hand and a right-hand part.
Scan your eyes across the music for a stopping point, such as the end of a phrase (usually indicated by phrase markings), section or the last couple chords.
To make things easier on the eyes, take a pencil and circle your choices for your cadences in the score. (Identifying cadence locations requires a lot of listening experience and a fair amount of theoretical knowledge when scanning a score.)
Let's say that the music you're looking at is in the key of C Major and you happen to see a G Major triad (or V triad, spelling G, B and D) resolving to a C Major triad (I, spelled C, E and G). When V resolves to I, we call this an Authentic Cadence. Since the tonic (C) is in the soprano (highest) voice of the I chord, it is called a Perfect Authentic Cadence.
This time around, you'll see the same thing you saw last time: V (G Major triad) resolving to I (C Major triad). You'll notice that the voicings are a bit different and that the tonic (C) is not in the soprano voice. When a member of the I chord besides the tonic is in the soprano voice, we call it an Imperfect Authentic Cadence.
Another cadence type you may come across is a little trickier. Still in C Major, we have V (G Major triad) resolving to vi (A Minor triad, spelled A, C, E). This time, since we expect the strong resolution of V to I, and we get V to vi, we call this a Deceptive Cadence.
Here, we see an F Major triad (IV, spelled F, A and C) resolving to a C Major triad (I). When IV resolves to I, we called this a Plagal Cadence; you may also be familiar with this sound because it is the "A-men" cadence in a hymn.