MSDN documentation of ASP.NET localization is good place to start reading about it. Basic steps are as follows:
- Localize your web pages - these would create language specific resources that will yield satellite assemblies for supported languages.
- Localize other aspects within application e.g. date/time formatting and input, images, user messages etc
- Set the culture and UI culture within your web pages as per needs (e.g. based on browser settings or user preference or user selection etc.). See this for how to do it.
See this walk-through that would quickly walks through basic steps.