http://blog.emanuelebartolesi.com/post/2007/12/Log4net-Simple-way-to-use-in-your-Aspnet-application.aspx

Introduction

Log the actions of your applications is very important especially when you develop new features or develop very difficult logical business.
But it is also important when users use your applications to understand the critical issues or problems.
To implement quickly the log operations Apache developed an opensource library for .Net developers.

Download

You can download the latest version of Log4net from this location.

Add reference to your solution

In Visual Studio 2005 select Project -> Add Reference.
In the tab Browse, find and select the dll Log4net.dll in your local folder.

Modify web.config

Into web.config file add this code into the section Configuration->Configsections:
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net"/>

Yet, add the section:

<log4net>

<appender name="RollingFile" type="log4net.Appender.RollingFileAppender">

<file value="c:\temp\web.log" />

<appendToFile value="true" />

<maximumFileSize value="1024KB" />

<maxSizeRollBackups value="10" />

<layout type="log4net.Layout.PatternLayout">

<conversionPattern value="%date %level %logger - %message%newline" />

</layout>

</appender>

<root>

<level value="DEBUG" />

<appender-ref ref="RollingFile" />

</root>

</log4net>

In this configuration will create a log file into the folder "c:\temp\" until its size is 1024Kb.
After this size the name of file will be web.log.1 until 10.
At this link you can find another configurations.

Global.asax

In the event "Application_Start" of the file Global.asax add this line:
log4net.Config.XmlConfigurator.Configure();
Begin to log

In every page you want to log something, add the static variable like below:
private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

This class has 5 levels of severity to log the operations:

log.Debug("log something at this level")
log.Info("log something at this level")
log.Warn("log something at this level");
log.Error("log something at this level");
log.Fatal("log something at this level");

Simple and fast to use.
You find the official documentation at this link.