Hi...
Not sure where to post this...

What do I need to set up a linux email server in the network and what type of linux operating system I need and email application that I will use

I'm a newbie

What type of hardware I need for email server.most of the pcs will access to the linux server are windos xp pro platform

It would be set up for a small business (Small Non Profit Agency) and would probably be set up primarily for internal use but would also need to be used with external mail also.

I want to try and teach myself as I go along with this project.

you've really said nothing about the size of it, if it's low usage, then literally *ANY* pc less than 10 years old can do that, with any standard version of linux. they can run an smtp server like postfix or sendmail and an imap / pop3 server like dovecot of courier-imap.