A disadvantage of digital advertising is the large amount of competing goods and services that are also using the same digital marketing strategies. For example, when someone searches for a specific product from a specific company online, if a similar company uses targeted advertising online then they can appear on the customer's home page, allowing the customer to look at alternative options for a cheaper price or better quality of the same product or a quicker way of finding what they want online.
I would focus on building your personal brand online and building a website right away that you can use as a testing ground for learning digital marketing skills. Most digital marketing programs are too theoretical and digital marketing is a fast evolving industries so you need to be constantly learning through active experimentation to become good enough to consistently make money.
Off page SEO: This type of SEO focuses on all of the activity that takes place "off the page" when looking to optimize your website. "What activity not on my own website could affect my ranking?" You might ask. The answer is inbound links, also known as backlinks. The number of publishers that link to you, and the relative "authority" of those publishers, affect how highly you rank for the keywords you care about. By networking with other publishers, writing guest posts on these websites (and linking back to your website), and generating external attention, you can earn the backlinks you need to move your website up on all the right SERPs.