First of all your 2010 mac cannot upgrade to Mojave, Mojave requires your iMac to be from 2012 or later.
So High Sierra it is.
If you have the installer apps for El Capitan and High Sierra you may find that they will not install as in October 2019 the certificates for several OS's expired. One way to get around that is to go into System Preferences> Date & Time
and reset the date back to a time before October 2019.
As you are on Snow Leopard at the moment you would need to install El Capitan first and then install High Sierra.
When the installer starts follow the prompts, at one point you will be asked to select a disk to install to, select the
partition/ disk you want to install to, not the Snow Leopard disk.
There are revised versions of El Capitan and High Sierra available for download with updated certification.
How to upgrade to OS X El Capitan – Apple Support
Go to Section 4 and click on Download OS X El Capitan
this will download InstallMacOSX.dmg to your Downloads folder.
Once that is done you need to unpackaged it, if you want instructions
on how to do that ask here, it can be quite confusing.
For High Sierra, open your Safari browser, Firefox will not work.
How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click Get macOS High Sierra,
this will redirect you to the App Store High Sierra download page,
it should say Get ir Download.
This will download Install macOs.app to your Applications folder.
