But there are some guidelines and points to think of. Firstly there is bountiful knowledge to be had in studying theories and formally approaching poetry and writing. Structures of cadence, rhyme, alliteration - all that good stuff. If you want a refresher in all that you can check out this pdf.
Like I mentioned earlier, what is particularly important in regards to rules is to place your own on top of the ones inherent to lyric writing. Being consistent with the principles of one's song lyrics makes them seem all connected and part of the same whole.
"Her eyes and words are so icy
Oh but she burns
Like rum on the fire"
The lyrical line implies a multitude of things about the nature of the melody that will emerge from them. Firstly, the line has inconsistent cadence and syllable amount per line. This will necessarily create a more fluid and variant melodic line - something which happens and continues to happen throughout the song: Cherry Wine (by Hozier). Also, the 'shape' of the melodic line will imply things about the way the melody develops throughout the rest of the song. And the tone of the lyrics "oh but she burns, like rum on the fire" are very particular. They're more casual than formal, but they are also very dramatic and create a vivid image. These aspects of the first three lines continue throughout the rest of the song.
Deviation from one's own rules - even the ones that weren't made intentionally - can (and almost always will) happen, but it must be acknowledged that this creates a strong effect which must be considered.
So this might all seem overwhelming, and in ways it is, but lyric writing can be mastered from any background. It's about deep thought and practice. Listen to your words, listen to the words of others, and above all - be honest with your ideas and don't be afraid to take risks.