How to get access to /admin panel without Devise?

I have question:

How to get access to /admin panel without Devise?
And how to create admin user?

I am afraid your question does not make any sense.
Does your application contain an admin panel?
What do you mean by an admin user? Do you know how to create a normal user?
Perhaps you mean that you do not know how to have users that have
specific roles. If so then look at the cancan gem.

If you are new to Rails then work through some good tutorials such as
railstutorial.org, which is free to use online. Then you will
understand the basics of rails.

Colin

Are you using the Active Admin gem? Active Admin bundles in Devise for
managing adminstrative users (which could be completely separate from
whatever your application does with user registration and
authorization).

Do you need to know how to get into it for the first time, so you can
set up users?

Do you have a current installation using Active Admin with Devise that
you cannot get into because you don't have credentials?

If you do have credentials to the application data base, you could
hack the users table, but I don't really recommend doing that.