I've been writing lyrics and poems for about as long as I can remember, and I recognize the need to learn the theories behind it. Itroitnyah did a great job of providing a summary of things to keep in mind.
My advice would be: Start somewhere. I prefer to do the chorus at first, since that's the part that's being repeated. The song should be based around the message in the chorus to feel "themed." Listen to the rhythm of the bassline or the lead, and try to write a simple sentence that follows that rhythm. Write the next sentence. Pull random nouns in and see how they sound in conjunction with the theme you're building. Wanna make rhymes? Do it. Don't want do rhymes? Go ahead, but beware that you're going to have to make the rhythms really stack up to compete against the catchiness of good rhymes.
Does line 3/4 not have enough syllables? Try to skip one, making it silent. Or drag a vowel out to cover the missing "word-beat" If used correctly, this can provide an interesting break and is mostly used to build up to a final line hammering the point home.
Next up, the verse. Keep your chorus in mind and use only the bassline as a rhythmic guideline. Try to keep all lines within a certain set of rules. Don't go ABAB Rhyme one half of the verse and ABBA on the second part, unless you've also changed the melodies in the second half.
Once every part of the lyrics have been written down and set up, we have a sketch and begin the polish-mode. Avoid overly repeating the same words and using the same rhymes. The start of verses can often begin with the same 1-2 words to provide a uniform feeling, but bear in mind that this mostly works in rhythmic-heavy music like Industrial, Metal and EBM.
Make sure you let the rhyming lines end on the same number of syllables. Otherwise it often feels like the song stutters. Try replacing "filler words" like "the, a, so" with something more interesting. Adjectives are almost always better use of a syllable.
EVERY SYLLABLE IS EQUALLY IMPORTANT, BUT THE START AND THE END COUNT EVEN MORE
Check sites like
Rhymezone if you're completely stuck