I am also relative new to investing. There is the Hang Seng Index tracker fund (2800) and the Hang Seng Index ETF (2833). Maybe there are others too but these are the two I know with my limited local knowledge.
You should be able to buy both of them easily through HSBC, you will have to pay a transaction fee. They both track the performance of the Hang Seng Index.
Not sure what exactly is the difference between those two, apart from the different lot size and the different price. PDLM mentioned somewhere that you would get dividend with the tracker fund (if you hold on to it long enough I suppose), not sure if that is also true for the ETF.
You can sell all or parts of your stocks anytime, but it will depend on the HSI if you can sell with profit or loss. Also, you will have to pay a transaction fee for the sales transaction. As far as I know that's normally around 0.2% to 0.5% including the government fees, depending on your bank.
You can use pages such as Google Finance, Reuters, Yahoo, etc to check the past performance out.
If you have stock trading enabled on your internet banking, you can simply buy and sell your stocks online. You enter the stock numbers, get a real time quote, buy at the market price, then you hold on to it. If you don't have this option in the internet banking, you can call phone banking and have them do it for you.