just installed 7.12 - but got stuck - how to access the backend and login
You won't be able to get the password from the database, but you will be able to get the email address for user 1 and then request a new password by email.

Have a look in the users table for the email address for the entry with a uid of 1. Then go to http://mysite.com/user/password, pop in that email address, and wait for instructions to be emailed to you.

After you've re-gained access to the site you can log back in again at any time by going to http://mysite.com/user. The admin backend will most likely be at http://mysite.com/admin, unless someone has changed it (this is unlikely).

The errors you got during install are because of a UNIX limit on the number of open files/file descriptors in the system. Take a look at this article for some suggestions on how to increase that limit so you don't run into the same error again.

It might be that you need to raise this limit and then re-install the site again...I'm not sure if the errors you've received will have stopped the site from being installed properly or not.