Put the following lines in your .cshrc file (and execute it)

setenv ROOTSYS /atlas/external/ROOT/v3.03.07/i386_linux24/root
setenv LD_LIBRARY_PATH $ROOTSYS/lib
setenv PATH ${PATH}:${ROOTSYS}/bin
alias root '${ROOTSYS}/bin/root'

Then type root  (this should work, I hope!)

For more root help, see the tutorial (pdf file, etc.) on 

http://www.pp.rhul.ac.uk/~taniamc/RootTutorial