How can I get my OS to upgrade to El Capitan without a credit card?

Hello,

I recently changed from Windows to Apple I-Mac and I am new to everything. My system currently runs Yosemite OS. I am trying to upgrade to El Capitan

but when I do Apps store wants a credit card #. How can I do this without a credit card #?

I feel forced into doing something I don't feel comfortable with. Please someone help!

Thank You,

Gene Overcamp San Jose Ca.