Have you searched authentication,security I have seen at least several topics on the issue. Do you have a role field??
See docs, email is example other fields are ok............

I 've seen them too, but I can't say I understood much...

I haven't changed anything, I have it as is by default, I don't know if it has a role field.

My 'users' table has no field named 'role', I' ve created a field 'is_admin'.

Learn about roles then accessing a logged in user, and that will get you there. I strongly suggest a seperate admin area.

I started with a separate admin authentication, copying (or more exactly trying to copy) the existing of laravel 5, but I failed totally :), so I thought I should modify the existing.

No need to modify anything, just add whatever fields you need in your user table, and do something like this:

$yourvar = $request->user()->is_admin;

And then do whatever if admin or whatever else if not an admin. You could use a redirect. A million possibilities here.