Arduino nano code for led decoration light .please help me.
Sure, no problem.
Welcome to the forum
Have you looked at the examples in the IDE ?
Take a new one, preferably genuine, out of its box, plug it in, LED flashes, job done.
If you can figure out how that's done, you'll be well on your way to some nice decorations.
Try the Blink example included in the IDE, understand what every line of code does, including forward slashes and semi-colons, and you'll be away.
Hi, @ye-min-thu_07
Welcome to the forum.
Can you please tell us your electronics, programming, arduino, hardware experience?
https://forum.arduino.cc/t/how-to-get-the-best-out-of-this-forum
Tom.... 
Hello, do yourself a favour and help us help you by reading first How to get the best out of this forum and posting accordingly (including code with code tags and necessary documentation for your ask like your exact circuit and power supply, links to components etc).