1-If your writing songs based on a chord progression, modes will not change anything. The chords formed on any given mode will be the same as the chords in the related major scale.
2-Where modes are useful is in writing single note lines to coincide with a chord progression (eg. bass lines, melodies, guitar leads). Make sure you know which chords correspond to which modes. An easy example is the bass line to Longview by Green Day, the first bar is based on the Eb mixolydian, and the second bar on the Db Lydian (notice the sharp 4th) because they correspond to the V and IV chords of the key (Ab).
3-If your not using extended chords (eg 9ths 11ths), it enables your melody parts to shift keys. For example when your playing an Am chord, the melody parts can work off of the Aeolian, Dorian or Phrygian modes all starting on A since the Amin triad is in all three. This way your able to use B (Aeo and Dor) or Bb(Phry), and F (Aeo+Phry) or F# (Dor) since they represent the 3 related keys that contain an Am chord (C, F, and G major, or Am, Dm, and Em)\
4-If your used to playing a pentatonic scale your almost there anyways, since these scales were created from the notes in common between the similar modes. The minor pentatonic is without a 2 or 6, so by just adding these to your basic pentatonic you have your modes (Aeolian 2 & 6, Dorian 2 & #6, Phrygian b2 & 6). The major pentatonic omits the 4 & 7 so replace them (Ionian 4 & maj7, Lydian #4 & maj7, Mixolydian 4 & b7).
Hope this helps.
Jeff