Music theory students in Theory I at CSUCI are studying SATB voice leading, counterpoint, and harmonization. There are really only a handful of rules to remember: (1) resolve chordal dissonances down, (2) don't double dissonance, (3) avoid parallel perfect intervals. Of course there are a lot of other issues to consider that eventually come quickly with practice, such as, spacing, correct notes & accidentals (especially in minor mode), direction of stems, key signatures, use contrary motion to avoid problems, use mostly stepwise motion to create smooth lines, create an interesting melody, add interest with embellishments, use the correct syntax, create different kinds of cadences.
These are the basics! Once you can do the basics, you can move to larger forms and work in different genres.