I don't think a CC has much to do with the green card but I do remember reading that some people had to show they could obtain a CC.
Secured means that you won't be able to spend more than the money you deposit in the bank and it will be kept by the bank in a savings account which you won't have access to until the account will be unsecured. In our case it took over 2 years and $ 1000.- isn't much to have as a secured CC. If you want to build up a credit history and get a good credit score you need to pay many times with your card and pay your bills on time...One late payment can ruin it.
We started with a $1,000.- secured card but soon we added another $ 1,000.- since when you are buying flight tickets you won't even be able to buy for the whole family and pay other stuff....since many accounts will let you pay the card of immediately but still the $ 1,000.- could be seen by the bank as the limit per month as i find out and it was very annoying. It depends on the CC and the bank...The bad thing is that most likely you won't find out until you use the card and you can even get overdraft fees if you use your card for over $ 1,000.- in one month....which is absolutely ridiculous since the bank won't allow you to spend more than that...In europe you card will be denied when you try to go over that amount...Over here it is all banking fees and you will find out when you get your statement. Many times we have figured out that the bank employees don't even have a clue how it works...If you call the bank 3 times you can get 3 different answers...