Really new to this platform and could use some help with how to upload a theme/template which I downloaded from ThemeForest. The files are compatible with HTML 5 and Craft 3 is on an Ubuntu server.
The answer will depend on the extent of your web development skills.
Do you have access to the web server via FTP? If so, upload the template files to the templates folder where you installed craft. The templates will have to be edited to bring in your sections, entries, and content that you add in the Craft dashboard.
Check out these docs to learning templating and converting templates to twig/Craft templates.
Do you have shell access (like SSH)? On an Ubuntu Server (and others), you typically have the following path to the html documents:
As Indigo Viking stated correctly, the details on how to do this vary based on your setup, i.e. your server setup, your virtual host setup on the server and your craft installation.
With Craft 3, there is a "templates" folder inside the "craft" folder where you could put your template. But in order to dynamically display content from the cms, you need to setup Craft in the Controlpanel with fields and sections and add Twig code to the template pages.
Checkout out this excellent (and free after registration) course on getting to know Craft CMS: https://craftquest.io/courses/craft-cms-3-tutorials