In my opinion, investing in yourself is the only non-negotiable investment you should make.
What do I mean by investing in yourself?
An investment is something you have or do that you believe will appreciate in the future. In the case of investing in yourself, you are appreciating the quality of your future self.
Here are a few examples of investments you can make in yourself: Read books, broaden your network, learn or develop new skills, improve your health, hone in on your nutrition, graft for your goals, take time off when you need it, foster relationships, journal, meditate, exercise... the list goes on.
The final point I'd like to make is a financial investment.
As much as I recommend saving and investing money in the traditional sense, I also strongly believe that financial investments in yourself are very important.
For example, over the last few months, I've been accumulating camera gear and lights which have helped my content to no end.
A few weeks ago I finally bought a vlogging camera so I could start YouTube.
These financial investments, should I follow up with the drive and action to make them worthwhile, will pay off eventually.
Your time is also money, and you are valuable - never forget this. Know your worth, know your value, and don't settle for any less.