To add CSS file follow the below steps:
Open File app\design\frontend\rwd\default\layout\page.xml Add the below tag for css: <action method="addCss"><stylesheet>css/[filename].css</stylesheet></action> Just below the last method=”addCss” add the above line. Now open, skin\frontend\rwd\default\css\[filename].css Start writing your css code in this file. To add JS file follow the below steps.
Open File app\design\frontend\rwd\default\layout\page.xml Add the below tag <action method="addItem"><type>skin_js</type><name>js/[filename].js</name></action> Just below the js include tags similar to the above line. Now open, skin\frontend\rwd\default\js\[filename].js Start writing your js code in this file. For Magento 2 please refers to this link.