when trying to install anything in my mobile n900 it always showing an error message unable to install!!

when i have tried the command apt-get update in x terminal it says could not open lock file/var/lib/apt/lists/lock- open 3 permission denied)

unable to lock the list directory

i have tried to download rootsh but when go to my documents to open it and open it with x terminal mnothing happens when write sudo gainroot

please could you help what this mean and what can i do to install thing???