how to activate plugin network-wide, and save setting across sites

I am developing a plugin, which will be used on single site, as well as multi-site.

How to make plugin active network-wide, code required.

Also, how to save settings network-wide?

Plugins can be activated network-wide by using the “Network Activate” link, or by placing the Plugin’s folder inside the \wp-content\mu-plugins directory.

For network-wide settings versus site-specific settings, I would look at how other Plugins implement such settings, such as WordPress MU Domain Mapping.