It's hard to explain but try and find out what mood the song is. For example, if it has a lamenting tone (like a ballad), upbeat tone etc.
Then try and figure out words that gel together, that fit the rhythm and mood of the song.
Or count the beats of the music and slot in any random words so at least you have a melody and know how many beats per line you need and go from there.
If you're writing a song that's personal to you, an expression of your heart, I know it sounds like such a cliche but just put all your emotions from your heart and see which words you come out with.
It's difficult to explain! Sorry if none of this makes sense to you at all. I just didn't really know how else to put it! It is such a difficult thing to do!
writing lyrics can be incredibly frustrating. i find myself spending weeks, even months at a time getting nowhere. i think it's important not to rush it, and be patient. inspiration will eventually come.
also, a rhyming dictionary can be invaluable. you can use online ones for free - i usually use one called RhymeZone.
i find it really helpful if as soon as i get an idea for a song, whether it's lyrics or chords, or even just a topic i want to write about, if i get a piece of paper or a notebook (preferably bigger, like A4), and jot down absolutely everything that comes into my head. then you have a solid foundation to work on later, and you can kind of piece your ideas together, almost like a jigsaw. also, if you have a sheet of paper with loads of ideas just scribbled all over it, like lyrics, chord charts or whatever, it looks really creative, which i think helps, haha!
recently, i discovered that i like writing lyrics in a cafe. the prospect of writing lyrics seems much easier when you're halfway through a latte! the downside of this, of course, is that i can't bring my guitar in, but i find the atmosphere very helpful for writing lyrics.
i hope this was helpful! writing lyrics is really difficult, but really good fun and rewarding. good luck, and enjoy!