I really won't dwell too much on this topic, as whole books are available on this topics alone! Performance testing takes time and patience to fully realize before and after results. To monitor your site's performance, use the Admin Tools' performance monitors and take the time to add the appropriate counters to the system monitor and take it from there.
Not only that, but .NET gives you ability to programmatically do the same things, alongside accessing system logs and system processes with its System.Diagnostics namespace. Read Displaying Performance Monitor Information through an ASP.NET Web Page for more info.
At any rate, read the book in the above link, and check out the article. Subsequently, Microsoft also offers a software tool that pushes your app to its limits, its WAS Tool. This tool and other useful information can be found here - Performance Optimizing Tools.
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