Will you be writing the whole song by yourself or are you in a band? Sometimes, it helps to map the structure of the song out first so you know how to tie the lyrics in with the intensity of where the track is going.
Write exactly what comes to mind. Write what you know. Write simply first and ignore trying to be clever or metaphorical. It'll all come to you and you'll eventually find your voice. Don't stress about it being god or bad. Just do it. And then keep doing. You don't have to show it to anyone if you don't want to or if it's not ready. Try to capture the mood and finish it while the sentiments are still pure. If you read it back to yourself and think "Wow this doesn't seem very lyrical." Fuck it. It's your poetry. And don't be afraid to suck. It's going to be awesome in the end and you're gonna feel fucking fantastic.