Do I need to go through all Mac OS upgrades? I right now have macOS Sierra, and want to upgrade to the latest Mac OS. Do I need to install high Sierra first?

Also, how do I get the upgrade offline files? And how do I install these?
The latest macOS is High Sierra. You can directly upgrade to High Sierra from Sierra with few steps. Follow the steps below:


Open the App Store app on your Mac.
Search the App Store for macOS High Sierra, or go directly to the macOS High Sierra page.
Click the Download button on the High Sierra page. If your Mac is compatible with High Sierra, a file named Install macOS High Sierra downloads to your Applications folder. (It's the offline installer file)
After download is complete, the installer opens automatically. Click Continue and follow the onscreen instructions.


If you are asking about Mojave, it's a beta and now available only to Developers now.

Source: How to download macOS High Sierra