The most common way to start Eglot is to simply visit a source file of
a given language and use the command M-x eglot. This starts the
language server suitable for the visited file’s major-mode, and
attempts to connect to it. If the connection to the language server
is successful, you will see the [eglot:project] indicator
on the mode line which reflects the server that was started. If the
server program couldn’t be started or connection to it failed, you
will see an error message; in that case, try to troubleshoot the
problem as described in Troubleshooting Eglot. Once a language
server was successfully started and Eglot connected to it, you can
immediately start using the Emacs features supported by Eglot, as
described in Eglot Features.
A single Eglot session for a certain major-mode usually serves all the buffers under that mode which visit files from the same project, so you don’t need to invoke M-x eglot again when you visit another file from the same project which is edited using the same major-mode. This is because Eglot uses the Emacs project infrastructure, as described in Buffers, Projects, and Eglot, and this knows about files that belong to the same project. Thus, after starting an Eglot session for some buffer, that session is automatically reused when visiting files in the same project with the same major-mode.
Alternatively, you could configure Eglot to start automatically for
one or more major-modes from the respective mode hooks. Here’s an
example for a hypothetical foo-mode:
(add-hook 'foo-mode-hook 'eglot-ensure)
The function eglot-ensure will start an Eglot session for each
buffer in which foo-mode is turned on, if there isn’t already
an Eglot session that handles the buffer. Note that this variant of
starting an Eglot session is non-interactive, so it should be used
only when you are confident that Eglot can be started reliably for any
file which may be visited with the major-mode in question.
When Eglot connects to a language server for the first time in an
Emacs session, it runs the hook eglot-connect-hook
(see Eglot Variables).