I like to use programs that give the equivalent of sticky notes. These notes come with Microsoft Outlook. If you don’t have Outlook, you can do a Google search on sticky notes. There are a number of free programs.
I keep going back to reqall.com its the most comprehensive, but if I want to create a simple todo list for say 1 day, wunderlist.com is fantastic. Both have free smartphone apps to sync. I like the simplicity of teudeux.com but no option for categories etc.