WARNING! Before you update your Operating system to Catalina ,there are a few things you must do !
1. Do a complete Time Machine Backup before anything else , just in case there is software you use that isn’t compatible with the new system .
2. Catalina will only work with 64bit applications , there are still a lot of developers that haven’t updated their software yet , so you won’t be able to use the older versions until they do.
3. DJ’s Just don’t do it , Catalina doesn’t have iTunes as such and the directories have been changed and a lot of DJ software isn’t compatible yet !
4. Its always good if you are going to update to wait until at least the first system update is out which will iron out a few bugs if any are there .
5.Turn off Automatic Updates if you are on High Sierra or Mojave , as they can sometime download overnight and when you boot up the next day i may have installed itself overnight.
6. Do you really need to update to this system ? Ive been running a beta of it for about 5 weeks and to be honest i can’t see anything of advantage to 95% of people ( i’m still on High Sierra on all my computers , no problems at all !
7. Here is a list of Software that may still not be compatible 
https://www.macworld.co.uk/…/apps-wont-work-catalina-36981…/
8. I can’t iterate this enough ,so ill say it again ,DO A TIME MACHINE BACKUP FIRST !