If you’re in your late teens or early twenties, it’s probably right to assume that you don’t have a lot of money. At least that’s what I’ve seen to be true for most people my age. And because our financial power is limited, we have to make sure that we know how to spend those pennies that we do have.

There are plenty of things that you can buy with limited amounts of money, and I will list some that I consider to be absolutely essential low-budget investments, that will, on the long term, help turn those pennies into 100 dollar bills.
Knowledge builds up exponentially. Being wiser also allows you to understand new things better and faster.
Books
Yes, books. I always talk about books, and there is a very valid reason for that. Books are the tool that you will use to go from an ignorant infant to a wise adult. If it sounds extreme it’s because the difference between well-read people and people that don’t care about working out their minds is evident, and indeed, extreme. Knowledge builds up exponentially. Being wiser also allows you to understand new things better and faster. Apart from it being an intellectually profitable exercise, you can also learn practical things by reading (provided you read books written for that purpose), so start building your own personal library right now, and don't be scared of designating a fraction of your budget for this purpose.
Courses
If you want to learn something but don’t enjoy reading books, there are also courses and seminars specifically directed towards almost any activity you could think of. It’s also a faster way to learn because you learn through practice, and in the case of seminars, you’ll also get to know other people interested in that topic, which might prove useful in the future.
Personal trainer certificate
If you are in decent shape, you may want to think about getting a personal trainer certificate. You can get one online for about 800 dollars, and it could really be orth it. Different than books, getting such a certificate will yield profit almost instantly (after you get one or two clients of course), but once you have it you can start billing a relatively decent amount of money per hour, which is always a nice option to have. You’ll have to study a little bit but it’s not extremely hard to get the certificate.
Invest in your health
Investments are not necessarily financial. There are some things that are more valuable than money, and your health is one of those. There is virtually no instance in which sacrificing your health for a few pennies saved is the right choice. Eat properly, join a gym, sleep well. Sure, you could save a bit more by eating shit, but is it worth it? Unless you absolutely need the extra money saved (which most of you reading this won't), it isn't. Don't be afraid to spend a little more in healthy food and in activities that benefit your health. Besides, getting sick is damn expensive, so try to avoid it by practicing a healthy lifestyle.
Be wise with how you spend your money, but also remember to think long term and begin building a foundation of health and knowledge that will be priceless as you grow older.