It's been some time since I opened an account. So my knowledge may be dated.
There are 2 US brokers with offices in Singapore
Charles Schwab Singapore (i.e OptionsXpress)
1 George Street, #07-01A
Singapore 049145
Phone for New account enquiries
(+65) 6692 7692
TD Ameritrade Singapore Pte. Ltd. (Also known as TOS i.e. ThinkorSwim)
1 Temasek Ave, 15-02 Millenia Tower, 039192
Phone: 6823 2250
I don't know how much $$$ you need to open account because I transferred my account from Etrade when they closed their offices. Just give the above 2 a call to find out what you need to open an account.
When you 1st open an account they will probably offer some freebies like free X number of trades. With only $500 you will not be able to us these freebies. Suggest you save up until you have more $ before opening an account. However if your intention is to buy & hold, then go ahead. You can gradually build a portfolio of stocks which may increase in value & provide a dividend.
If you are a seasoned trader & regularly trade you can consider Interactive Brokers which is based in Hong Kong. They charge low fees to trade. However you need to be a regular trader because this company will charge you a monthly fee if you do not satisfy their monthly minimum requirements. They are here
https://www.interactivebrokers.com/en/index.php?f=1590
With ThinkorSwim you can try out their trading platform without committing any $. You can do paper trading on a mock ThinkorSwim account. It doesn't cost anything to create a paper trading account. I don't know if the other companies offer a trial version. TOS has many online courses which you can access if you want to learn to trade. There are also Youtube movies in how use their platform.
Be aware that the companies which have offices in Singapore have to satisfy MAS(Monetary of Spore) regulations. In the questionnaire you have to fill out when opening an account there will be questions on whether you are an experienced trader. If you indicate little experience then you will have to attend an online course. If you want to avoid this hassle, just indicate your vast experience : )