The content below are gathered from the internet accompanied with my experience. I will appreciate if you have any feedbacks.
Enjoy your Umbraco journey.
How to Debug
Setting trace in default.aspx and web.config
Log in Umbraco
assembly: businessLogic.dll
namespance: umbraco.BusinessLogic
class: Log
log file is in App_Data/Logs/
How to Add Umbraco Extension
Data Presentation
<xsl:when test="$size >= 1024"> <xsl:value-of select="format-number($size div 1024,'#,###')"/> <xsl:text>KB</xsl:text> </xsl:when>
Template
Use jQuery to load a page with another template