Do i always need to start the new site in django by "django-admin"
No you don't need to use startproject.  You can copy files from another project or write the skeleton files yourself.  You probably want to have a unique SECRET_KEY setting though.
When i need to start new django i use   django-admin startproject  site1 and it then create skeltion files.
Then use syncdb
i want to know if just copy the files from previous site e,g site1 and then rename it to different folder and make necessary chnages in settings and URL and then use syncdb . Will my site work or
I have to use startproject always and it does some database inserting other than making skelton files