I just finished A levels, I only have $500 savings. How can I start investing?

Yes. I fully agree with Justin. I would say your best choice is Invest in knowledge. If I could go back a few years, I would have fully utilised the time I had post As to learn more- be it developing hard skillsets like coding or reading more books on finance and investing.

Alternatively, pre-army i would recommend pick up a good part time job that pays decently well(8/hour?)

Army after BMT and SCS/OCS will be your best friend. Better still if you are pes C or E. Use that time wisely. When you are ready, you would accumulated a sizable sum of savings(part time job + first few years in army) to make signifcant enough of a portfolio. Without the capital or knowledge, one without the other would be pointless.

Besides. There aint nothing better staying up for guard duty and being able to monitor US markets at the same time.

Since you just finished your A levels(18-20yrs) , I would suggest that you invest in yourself first. Get new skills, knowledge, experiences, etc. Those will help you in the long term.

Once you have gotten a job and have a more solid capital/inflow of capital, then would be a good time to invest.