how to use plugins in the web game?

I want use Plugin in the web game but I do't know how to do it.

Use of plug-ins in Unity are documented here. There is a note right at the top of the page saying:

Note: Plugins are a Pro-only feature. For desktop builds, plugins will work in standalones only. They are disabled when building a Web Player for security reasons.

If you want to use Mono assemblies (that also use a .dll extension) that's a different matter. They can be included just by dropping them into the Assets folder.

If you're talking about native (c/c++/objc/etc) plugins, then you can't do it - It would be far too much of a security risk for it to work