erase

Hi, A friend of mine recently gave me her old iphone 3GS to help her backup her photos. Note that the phone no longer has a sim card inserted. I'm not familiar with iphones, always had android phones, so I charged the battery, plugged the iphone to my laptop. Couldn't access the files directly...

I want to gift my iPhone to my niece along with all the music I put on there, over a thousand songs. But I want to delete everything else, contacts, pictures, passwords, browsing history, my phone number, banking and credit card info, etc. Is there an easy way to do this?