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"
/>