Please tell me what should I do to login in Ubuntu with administrative rights

I dont think you can log in to an entired session as root, however you can become root if you issue the command sudo -i
in a terminal
Good luck to you!

For resetting a forgotten password and other similar tasks hold down Left Shift and select the item with "(recovery mode)" after it. For anything else precede the command with sudo, for example:If you wish to enable root login it can be done but please see the following page and read it carefully:
https://help.ubuntu.com/community/RootSudo

The first user you created when you installed Ubuntu is automatically in the 'admin' group. But you can add other users to 'admin' group as well, which makes them administrators with the right to use 'sudo'.

More details here: http://www.psychocats.net/ubuntu/sudo

As implied by TroN-0074, Ubuntu gives the first created user full root access rights via the 'sudo' cmd, so just prefix privileged cmds with that.

Enabling the Root account is rarely necessary. Almost everything you need to do as administrator of an Ubuntu system can be done via sudo or gksudo. If you really need a persistent Root login, the best alternative is to simulate a Root login shell using the following command...

To enable the Root account (i.e. set a password) use:

Logging in to X as root may cause very serious trouble. If you believe you need a root account to perform a certain action, please consult the official support channels first, to make sure there is not a better alternative.

Straight from the Ubuntu website.

The OP asked nothing about root user, the question was administrative user. Administrator has a specific meaning in Ubuntu, it means a user who is in the 'admin' group and is therefore authorized to administer the system using 'sudo'. Must we turn this thread into another tired "don't log in as root" debate?

Indeed, you are right.
It's not our fault though that Canonical decided to go their own way, thus confusing Linux users.
I think I'll stay away from Ubuntu threads in future. Sorry I know that's negative and OT but it really is turning into Ubuntu OS now.

Since the first user IS administrative, and he mentioned a LOGIN, it could be assumed he MEANS root, and just used "administrative" because he's a crossover from Windows, since he can already log in as an administrative user.

Ubuntu calls this group 'admin', Debian calls it 'sudo', and Fedora calls it 'wheel'. Since there is no standard terminology, someone is bound to be confused no matter what.