Exchange server installation problam

i am working in a componey
i have to install aexchange server for this componey
but i have no idea
the componey has upto 150 employees localy in head office and
also some upto 50 employees in 3 branches.
plz guide me for this trouble.
i have a little idea about exchange server
for example it is used for sending and reciving maills within the componey
but i have no idea that,can it use to send and recive mails from and to outside word?
i am using wind 2003 server
so i need fulll cooperation in this problam
secondaly i want chat dierectly with some expert to clear my problams.
thanks