hello, I am new to linux and the first problem just popped up.

First i installed ubuntu, and then i searched for my ati 9250 driver. I did the installation by a guide and when i completed i have to do some configurations.

I made a mistake by chosing an option which was about the type of my keyboard.

I restarted linux and when the login screen arrives i cannot change the keyboard language.
The first option is english and the second is greek.

My username is george but i cannot type in english the alt+shift doesn't work and automatically chooses the greek.

Please anyone can help me?

Thank you in advance.

This is how I would try to fix it. I have no clue if this will work, but it's worth trying. Try pressing [ctrl]+[alt]+F1. This put you in a virtual terminal. The keyboard settings should be normal outside of X11. Then enter:
to fix your settings or edit /etc/X11/xorg.conf if you're feeling adventurous.

good luck.

Maybe this is what you are looking for:

setxkbmap -layout us

I'm somewhat confused, though, as you seem to be mixing up two things: layout and language. I assume it's the first you're after.