If I may, I have been a (semi successful) musician/song writer for many years. May I offer a little advice?
I'm not sure what kind of a song you are writing, fast/slow,
radio friendly/avant garde, country/rock but here are some basic guidelines
When you start to write a song most writers start with what's called "the hook". The hook is the catchy, often repeated phrase or riff that "hooks" the listener into the song. This hook could be anywhere in the song, but it is more likely to be the first line of the "chorus"
examples. "relax, don't do it", "pour some sugar on me"
"You got to know when to hold em"
The hook is often a clever play on words (but you don't want to get too cute) example: "Stop in the name of love"
I'm not sure how advanced you are in song writing, so I'm sorry if I'm pointing out the obvious. One last thing, I don't want to be long winded.
Song structure (very important)
There are MANY ways that a song can be structured, but when first starting out, it's a good idea to stick to a basic format until you become comfortable with lyric writing. The basic form is this.
1. Intro music (optional) the intro music is often the chords/riff that comprises "the hook" (with no lyrics)
2. Verse1 (usually 4 lines)
3. Verse 2 (usually 4 lines)
4. Chorus ( usually 4 lines with "the hook/lyrics as the first line
Often times the first and last lines are the same (the hook)
5. Musical break containing a solo (guitar,harmonica keyboard
6. verse 3. (4 lines). Often used as a story closer for the first
2 verses, OR contains a surprise ending.
7. Chorus ( often but not always identical to the first chorus)
8. Repeat chorus.
If this has been helpful, I will be glad to offer more advice.
It would help if I knew the type/tempo/genre of song that you are trying to write. Good luck.