I have an iMac(21.5, Late 2009) and want to upgrade to OS Catalina

Only 2012 and later iMacs can upgrade to Catalina. See: How to upgrade to macOS Catalina - Apple Support

You can't. That model is not compatible with Catalina, particularly the graphics card. The highest it can go is to 10.13.6. So start saving your money for a new iMac. FWIW I'd stay away from the 21.5" models because you can upgrade the memory yourself whereas with a 2u7" model you can for less than ½ of what Apple charges. Just some food for thought.