<human>: I'm starting to plan how to propose to my girlfriend, and a long time ago she mentioned that when she was younger she fantasized about having a rock star propose to her in song (was friend-zoned at that point).  Now, a couple years (and defriend-zoning) later, one of our inside jokes is a really simple but annoying jingle she made up that often gets stuck in my head.  Given that it's catchy, I decided to try to turn it into a song, if it turns out well, I'll see if I can make that little girl fantasy come true.  
Now here's the rub,  I just started playing piano last year.  I'm not terribly good, but I have a decent grasp of how to use the thing.  I also took some classes on music theory a long time ago in school, so I have a passing knowledge of how music works, chords, progressions, keys, reading music, etc. 

Now, with that said, I am completely new to composing music from scratch, and though I also have a bit of a poetry background from college, I've never written song lyrics either.  So, Reddit, I know there must be some of you out there who are good at this sort of thing, and some of you probably even make your living doing this.  What are some common and stupid mistakes I should avoid making?  What are some things that newbies make?  Is there anything I can do to make my end product sound more professional and flawless?

I do have an incredibly talented and knowledgeable music teacher who is helping me out, but I don't get to see them very often, so most of the onus is on me.  I really need your help, Reddit!

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: I'm writing a song to propose to my girlfriend, and I need your help!  Also, read the title.