First read this http://tomcat.apache.org/tomcat-7.0-doc/config/valve.html and http://tomcat.apache.org/tomcat-7.0-doc/api/org/apache/catalina/valves/AccessLogValve.html
Add something like this to your context file (also can be added to Host)
<Valve className="org.apache.catalina.valves.AccessLogValve" directory="${catalina.base}/logs" prefix="access_log" fileDateFormat="yyyy-MM-dd.HH" suffix=".log" pattern="%t %H cookie:%{SESSIONID}c request:%{SESSIONID}r %m %U %s %q %r" />