SD card "write protection"

I am very confused about my sd card. it's 2 GB and when I insert it into my mobile phone and was deleting some files so it shows that all of the folders contain "write protection". the option was saying that firstly remove write protection then delete these folders. Then I go for formatting this sd card so that it will be easy instead of deleting the folders one by one but it also didn't help it was demanding pin number and I don't remember the pin number because this sd card was used by someone before it is not a new one. Then I search on the internet it says that check whether your pen drive has a lock or not. I don't know how to check it I am very confused about it I couldn't use it :( please help me for this