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"
/>