Can not update from High Sierra to Catalina

How can I update from High Sierra to Catalina. I tried to update to Mojave first but it keeps saying that I need to download Mojave 10.14 but I can't seem to find it anywhere on apple website. I downloaded and tried to use all the different versions of Mojave and all of them said I need to install Mojave10.14 first. Ultimately I just need to update to at least Mojave. Can anyone give me directions on how to update my MacOs.