it depends on what kind of poem
for a song, start with a chorus, and build around that

Well, I personally, along with probably a lot of other people, think that what makes a good song or poem is the feeling you get while reading or listening. TIhe best thing you could do is to first try to decide on an emotion or message that you want to get across. Once you do that, its just finding words or beats that bring out that feeling or message. It's hard, but choosing just one thing to focus on can really help and sometimes the trial and errors that come with doing it can lead to something really cool even if it want you intended path. Don't push too hard to go a certain way, just do what you feel is right and kind of go with it? If that makes sense I don't know its like, 4 in the morning ;p

Like above, try to think of what the poem/ story has to be about. What kind of poem will it be? What type of narritive?
But also, write about what your passionate about.
Like, when I took creative writing, I was always writing about my OTP. Id change names, sometimes switch roles, but in the end, it was always the doctor and rose. Write about what you care about. Change it to the circumstances, if you have to, but if you don't care, its not worth it.
Hope this helps! :)

I've written lots of poems before so I have only info for that. Tbh you really only make the best ones when your in a certain mood. Something happened or you thought of something that got to you.
You don't need to be specific in what your trying to say, but make sure it's still about the same event/feeling and your flow is smooth, meaning don't take a long break when writing and definitely don't leave it for another day otherwise your heart won't be in it and can't make a strong poem. Write as much as you can in one go and then make some changes after. Your initial style remains and that's important.
Stick to that one event or feeling. Use short and long sentences, some rhymes, some random, you can just place words rather than a sentence so to give an effect, and you can repeat words.
The rest is all up to you. Write what you feel, and don't hold back at first. Changes come later.
Feel free when you write. Don't worry about what others say.
Good luck with it.

Thank you all for the advice:)