THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
First read this http://tomcat.apache.org/tomcat-5.5-doc/config/valve.html
Add something like this to your context file (also can be added to Host)
<Valve className="org.apache.catalina.valves.FastCommonAccessLogValve" directory="${catalina.home}/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" />