Originally Answered: I want to write a song, but I need ideas?
Lets beggin by making a statement: The most important part of a song is the melody you sing and what you say when you sing it. So, I always recommend my students to focus on the sung melody, rather than on anything else.
Rule#1. What to say: the theme of the song
It is crucial to bring two things together. Spoken rythm and sung melody. This marriage is very important. It is the core of any good song. So, think of one phrase that you would like the song to say, and then start trying different rythm and melody patterns on it. Use the guitar to find proper combinations if you need to. Dont focus on the chords just yet, we´ll do that later. Just strum a single chord and focus on getting one melody to go along with a single line of lyrics. Once you have something you feel confortable with, you have a theme for the song, and we can move on.
Rule#2. Musical elements: What to use.
I find useful to identify one musical element and then make use of it throughout the whole song, in different ways. I mostly use the guitar, so in my case(and in yours apparently), this element could be an arpeggio, a strumming pattern, a certain use of chords(open, add 9th, minor...), a riff, etc.
Look for an element that goes well with your theme and then start developing the theme into melodies with lyrics. This will get you ready for the next step.
Rule#3 Put it all together.
Whats first, music or lyrics? In my opinion, lyrics first, but very closely after, comes the music. Its like this: hopping with one leg is harder than just using both to walk. One step music, one step lyrics, so it all comes together well.
Some people find useful to have a bunch of related phrases (lyrics pool) ready beforehand, so when the time comes to make melodies, they can fit any melodic idea to an already written phrase, from the "lyrics pool". Some people have the talent to come up with both melody and lyrics at the same time. This is very personal, but the goal is always the same, to develop a chorus, a verse and (not all the time) a bridge.
Once you have this, you'll be almost done.
Rule #4 Chords are not as important
The chord progressions are not as important and can be modified "on the run", while developing the theme (or themes). Just use a basic chord progression, and then, after the song is ready, you can modify it to your taste.
Chords (and overall harmony) to music are like dishes to cooking. You can prepare any recipe and then serve it, from the cooking pan to the serving dish. Depending on whats your purpose and what the ocassion is, you can serve it in a square white large dish, or in a round transparent dish, or even eat out of the pan. Chords work the same way. Once you have the song ready, you can look for better chord progressions to fit your melody and lyrics.