Agree completely with Juinn. Digital Marketing has a lot of components and each plays their own role in an integrated marketing (both digital and offline).
Generally speaking, it is easiest to start with social media marketing such as Facebook marketing because of the low entry barrier - it is cheaper, and more user friendly, even if you are not an expert in creative. Also because we are all Facebook users, so we can imagine the end results.
That said, there are many other marketing tech that sort of consolidates all these ad serving platforms into 1. I'm conscious about using Marketing and Advertizing because most people think about "Advertizing" when they say Digital Marketing. But "Marketing" is much more than just Advertizing.
Apart from Google Academy and Facebook Blueprint Some platforms like Marketo, Hootsuite, and Hubspot has quite a good overview on the different components and how they are used. You can also look at other Youtube videos on specific topics once you're familiar with the basics.
Among others, here're a few that I think are the bread and butter of Digital Marketing:
1. Email Marketing
2. Search, Adwords, Search Engine Marketing (SEM)
3. Google Ads, Google Display Network
4. Facebook (Instagram, Youtube, LinkedIn, and other social media) marketing
5. DSP and ad serving platforms
6. Data Management Platform
7. Programmatic advertizing
8. Search Engine Optimization (SEO) (don't mix up with SEM)
9. Web analytics (Google Analytics)
The best way to find out more would be to go through the online resources to get some basic understanding, then talk to as many digital marketeers as you can. Each will have their own insights and strategy. Finally, try to incorporate digital elements in your current marketing role.