I'm a songwriter, and so is my little sister. But I'll tell you what I told her when she asked me the same thing, many years ago.
Songs have to have your voice in them. The best songs are about things we feel in our own lives, and making other people feel the same way we felt, when we felt them. You should try to find your own voice, or your own style, and not really try to care too much about what other people might think, or say. Sure, we play our songs so that other people will hear them, but the one person who matters most, when it comes to judging the song, is yourself.
If you have trust in your musical and poetical tastes, then you will know a good song when you write it. And you will be confident that it will reach out to at least someone. Not everyone will like it, because not everyone has the same tastes. If somebody puts you down for your songs, its probably just because its not to their tastes, so you can ignore them.
I hope this helps! Have fun!