Im trying to write a song but i always have writer block and sometimes it just comes to me but i forget it?

Do you play an instrument, or know someone who plays one and is willing to jam with you? Some people have a better time writing songs if there's a melody. I find guitar players are best for this, because anyone that's good at guitar can pick up what genre and tone you want for the song. Also look up how a song is structured for your genre of music ( like verses, hooks, concepts, chorus) this will allow you to craft a better song. Look at websites that show lyrics, you can analyze how others wrote their songs.Look back on oldies for inspiration, songs that made you want to sing in the first place. Write what you know, this let's your listeners feel something that's not generic commercial ear noise. Finally keep a notepad, nothing big or fancy, with you so you can jot down lyrics as they come to you where ever you are.

Try carrying a small notebook around with you so you can just jot down whatever comes to you when your out and about. I'd need to know the tune your putting the lyrics to to decide on puncuation.