Now that you have the right state of mind, are you having trouble getting the format right? Here are some tricks to help get it written.
1) Start generalIt makes it difficult to write music just off the bat. If your sitting brainstorming things to put in your song, write them down. Just bits and pieces! Don't make it too extravagant because it's just ideas. With your writing material whatever it may be, jot down the lines and words that pop in your head. Don't worry about what to rhyme it with just yet, that will distract your brain. Focus on the main ideas.
2) Go through what you haveTake some time and read through the ideas you have written. Think of what will sound good in the chorus and what will be the hook. This doesn't mean you have to use ALL of the ideas. You can even change things as you need to. It's what you want and nothing can contradict that.
3) Put it togetherPoem form is usually the easiest way to look at it. A song is basically a long poem. If you look at in that way, you wont have to worry too much about the music yet. It lets you concentrate on the actual lyrics.
This a a short little bit of help. Hope you get some solid structure with your work. Remember, this is just my outlook on things and what I do that helps. Just spreading the knowledge.:)