well if you are fully new to this phone then follow me
first of all read preenv FAQ as marmistrz said
and read read then read or else you will not understand anything follow this anyway :
open your application manager and enable extra extra-devel extra-testing catalogues just get it from google you will find it easly then install rootsh from application manager then go to xterminal app in the app menu and type :

root
apt-get update
apt-get upgrade
apt-get install preenv

if got libgles1-sgx-img missing for preenv download it from my post here http://talk.maemo.org/showthread.php...83#post1168783
then download "AssassinsCreed.deb" from (demonoid.com or any other one mediafire.....etc)
and when you want to install a (.deb) file
connect your n900 with pc and pc the file on your device outside any folder then unplug the usb cable then go to xterminal again and type this codes :

root
dpkg -i /home/user/MyDocs/ (file name ) then press enter

Notes
1- dont open x terminal and application manager together
2-press y when x terminal ask you while install a package
3- write capital letters as like this (MyDocs) in codes because this is LINUX!!! Not WINDOWS
4-press tab on x terminal screen to autocomplete files/folders names just like (dpkg -i /ho "press tab use "press tab MyD "press tab " then at least first 5 letters of the file name then " press tab " wow the enter
5-if got the assassins creed in (ipk) extention then that is another issue (if yes you got it on (ipk) tell me to give you another guide )

that is all

my advice to you is start reading about your new phone "THE GRAND N900" or else you will get lost
Here is the catalogues ........