Use the org.eclipse.lsp4e.languageServer extension in plugin.xml to define a language server (ie how to start it and connect to it) and to associate it with one or many content-types.
See multiple examples in the test.
Use the ProcessStreamConnectionProvider if your LS is accessible as a process.
To handle LSP Commands (e.g. associated with CodeLens messages) in the IDE, register a sub-class of org.eclipse.lsp4e.command.LSPCommandHandler in the plugin.xml file of your plug-in via the org.eclipse.ui.handlers extension. The handler has to be registered for the commandId equal to the Command.command attribute of an LSP message. Note that client side handlers will not be triggered, if the language server is capable of executing commands of the same commandId.