For an international student like me in the US, what are the ways I can invest if I can just save 50$/100$ a month? Is it too early to think about investing? I am 27 by the way. 
It’s never too early! I would of course pay off any debt you have first, then start investing early and often. I don’t know all the tax implications of your situation, but something simple like opening a Betterment account and dumping your $100/month in there would be a great start. Just keep ramping it up over time and plan to build wealth over decades! 