How to erase memory card?

This depends on how seriously you want to erase them. If it's just that the card is full, having copied all the pictures you want you can either reformat it in the camera (it'll probably be somewhere under the Settings menu) or you can plug it into a laptop or PC (using a USB card reader if necessary) and simply delete the picture files there. However, they may still be recoverable with a program such as Recuva (https://www.piriform.com/recuva).

If the photos include anything embarrassing or incriminating that you regret, and you want to be sure they're gone (hey, we all do a few stupid things in our lives), delete all the photos and other files with the card connected to a PC, then download and run h2testw from http://www.heise.de/download/h2testw.htm... This will overwrite all the free space, which should now be the entire memory card. There remains a chance that a forensic analysis could still recover a small proportion of the data, so the only way to be really sure is to smash the memory card into little pieces with a big hammer!