Solution1 (quick fix) : Add below code in your JSP
<script src="<%=renderRequest.getContextPath()%>/js/somejs.js"></script>
<link href="<%=request.getContextPath()%>/css/mystyles.css" rel="stylesheet"/>
Solution 2 (Liferay recommended) : add below code in liferay-portlet.xml
and let Liferay load the css & js files.
<header-portlet-css>/css/mystyles.css</header-portlet-css>
<footer-portlet-javascript>/js/somejs.js</footer-portlet-javascript>
But i would suggest keep your all js and css file into theme and access from there