For creat a song ,so what to do for this purpose
Answers
The options here are limitless...
To put it the "artistic" way, simply relax and let the muses speak through you. Find somewhere comfortable you can be alone with your instrument or some instrumental recordings and a pad of paper/pen (depends on what you're trying to write) and just start playing/writing away until you happen upon something that catches your ear. It can be a single chord or the whole chorus section; it doesn't matter so much WHERE you start, more of the fact THAT you start. Remember, every tree started as no more than a tiny seed!
I've found that chemical "inspiration" of various kinds can help CERTAIN people to relax and just let it flow. Other people don't seem to have as much benefit and just seem to do better without. I suppose it's up to you to figure out which you are. Personally I find sedative-type things can be fantastic for erasing that self-doubt and really generating a lot of new ideas (so many, in fact, that often you won't be able to remember most of them unless you're recording yourself).
Regardless of where you find your "seed", after the bulk of the actual writing is done it is usually best to edit/arrange your song in a clearer state of mind (although I say do whatever works best for you- always). From there it's a matter of making everything flow and become one cohesive, single piece of art.
Some tips:
John Lennon allegedly once advised George Harrison that he found it easiest to get all of the "writing" done at one time. Take that for what you will...
Music theory and the works of others can be great places to look if you're lacking inspiration or just hit a wall in writing.
Sometimes taking a break and walking away/doing something else for a while can help to get you moving again when you're struck with the dreaded "writers block".
Always trust your ears when writing. Don't play or write something a certain way because somebody else says it does or doesn't sound good. It's YOUR music, nobody else's (Of course this doesn't apply when writing with others).
I could probably go on and on and on (I might come add to this over time), but for now that should be plenty to help get you started! Good luck!
Feel free to send me a link or copy of some of your work. I'm always happy to help critique others! Or just share their music with the world :)