One thing that's important for any person that's just starting in the world of digital marketing is to have some level of "independence" from other professionals. That's extremely relevant because you can't always rely on having technical people around you (or won't have the budget) and you need agility to perform tests and implement ideas by yourself.
Of course, one day you'll have an entire team to help you with each one of those, but here are the skills that I believe any digital marketer must have (in no particular order):
- Copywriting / Webwriting: writing well by far the most important skill that you need to have. Landing Pages, Blog Posts, Videos, Social Networks Updates, PPC Ads and basically everything that your audience consumes depends on it.
- (Basic) HTML and CSS: you don't need to be a programmer, but a basic understanding will save you a lot of time when you need to do adjustments on websites, find nasty bugs, implement scripts or do some research on competing websites
- Image editing: you don't need to be a photoshop expert, but you must know how to change image dimensions and formats and add overlay graphics and texts. There are some very good tools to help with that, like Canva
- Wordpress: it allows you to create multiple types of websites with no coding experience. Do you want to test an idea for a Landing Page? Create a Blog? Corporate Webpage? Social Network? E-commerce? Reddit-like site? Almost anything you can think of can be easily done with Wordpress.
- SEO: you will definitely need those sweet organic clicks, so you must know how SEO works, both on the technical and content levels.
- PPC Advertising: unless you're a content marketing purist, at some point you will need to accelerate things through AdWords, Facebook Ads or any other Ad Network. You need to be familiar with the way they work (the concepts are the same for most of them) and be able to generate results.
- Spreadsheets: yeah, there's no magic or guessing in Digital Marketing. You'll need to gather, process and understand a lot of data and the fastest and cheapest way to do this is using Google Spreadsheets.
If you have the skills listed above then it will be much easier and faster for you to do the "strategic level" stuff like A/B testings, campaigns, etc. Those were all well covered in previous answers to you question.
Good luck!