OK, from my vast experience and advanced years, I have some advice I wish I could have given my younger self, which is this...
What is paramount in a song is the voice.
When you write a song, start with your voice. If you find something to say, that's 99% of the song done and dusted. (When I say "find something to say", I mean something like "We will rock you" or "i bet you look good on the dance floor" or "love's the greatest thing" or "you are the sunshine of my life" or I wanna hold your hand" or whatever little phrase comes out of your brain.)
I'm making an assumption here that you're speaking to a rhythm (otherwise it's something commonly known as "talking".)
So, get a metronome or a simple drumbeat, and put some words to it.
If it's cheesy, you'll know it. If it's crass, if it's pretentious, if it's inauthentic, if it's overly sentimental, bombastic, maudlin, etc etc, you'll know it. Assuming that is that you can hear anything you have to say and judge it without prejudice in your favour, on account of the fact that you are you, and we all have a tendency to want to be better than we are, and have to fight that tendency in order to arrive at an honest appraisal of our talents.
(Of course, that is perhaps the hardest thing of all, to find that balance between being too hard on ourselves and being too generous, which is why all artistic creation ultimately comes down to a question of judgement.)
So, when you have some words sung at a particular pitch (or different pitches if you're really having fun), take an instrument and see what notes fit over it. Then you have a song, or the kernel of a song which can be expanded on much more easily than if you were to start from scratch.
But that point about having fun is important too. The greatest art is really the one where we can see that the artist has had the most fun.
Imagine if someone knew they were going to die in five minutes and chose to spend those last five minutes making music. Who the feck would do that? Boy oh boy, they must really love producing music. What kind of music would someone like that produce, that they would like to spend their last moments of life making music? At a guess, I'd say they were either crazy, self-deluded, or really having fun. It's the latter kind of music that I'd be really interested in hearing.
OK, I'm starting to go off piste a little.
To get back to my original point, a song more than anything should be built on the vocal line. That's pretty much the definition of a song.
So, as an exercise if nothing else, see how far you can get, see what you can achieve, by starting that metronome and putting down vocal phrases - no guitar, no piano, no bass.
Sorry, didn't mean to go on so long. I suffer from a serious medical condition experts sometimes refer to as longwindedness.