$500 is actually a solid amount to begin with - the real question isn't whether it's enough, but whether you're ready to leave it alone for years. Most people mess up not by investing too little, but by panic-selling during a dip or constantly tweaking things. That said, Fidelity or a similar low-cost brokerage works fine; fractional shares mean you can buy into index funds immediately without worrying about hitting a minimum.
Here's something practical that others might skip: before you move the money, spend a week or two actually *using* the app's interface on paper or a simulator if they have one. Seriously. You'll figure out if you understand how to buy, what the fees actually are, and whether the UI makes sense to you. I've seen plenty of people open an account, get confused about something basic, and then lose confidence. Spending zero dollars and ten minutes on a dry run saves so much frustration later. Also set up automatic transfers - even $50 a month - if your budget allows it. The compounding matters less at first than the *habit* of actually investing regularly instead of moving money around based on mood.
If you've got no debt and stable income, you're in the easiest position to let this grow. Pick a simple index fund (something tracking the whole market), set it and forget it. That's genuinely the winning move, not because it's boring, but because boring works.