How do i put a song on my youtube video, that i made in flash without copyright issue



well...you could use windows movie maker and just drop in the audio track and then render it together. personally I prefer Sony Vegas for editing but movie maker will do the trick.

or if you don't have a particular song in mind, you can just upload the video and then browse the free music clips that YouTube provides and it will automatically integrate it with your video.
 
Actually come to think of it I don't think Sony Vegas supports flash video, and I wouldn't be surprised if movie maker doesn't as well. You might need to convert it to AVI or something first. There are a bunch of free converters out there.