A little general harmonic advice for starting out:

from http://www.kvraudio.com/forum/viewtopic.php?t=164330

Roots of chords a fourth and a fifth apart generally sound good. 
Roots of chords falling a third is good. 
Roots of chords rising a third is good strong to weak, but bad weak to strong. (In 4/4, the strongest beat is the 1st, then the 3rd) 
Don't sandwich chords (ie, have at least 2 different chords before having the same one again). 
Try to use a mixture of root position and 1st inversion chords. (Avoid second inversions) 
Avoid diminished or augmented chords (eg. chord VII, and chord II in a minor key)
Avoid chord III. 
Avoid the progressions II to I, and V to IV. 
Avoid the bass note of a chord on a strong beat the same as the bass note of the preceding weak beat. 
The top line and the bass should not approach an octave or 5th in similar motion, except by step in the top part. 

The bass is also very important. 
It should generally move by step whenever possible, and should not leap around all over the place. 
Contrary motion between the bass and the top line is good. 

My posts on Voice Leading, and my Introduction to cadences might also be useful to you.