please can anybody help meout, i am new to linux, but i am giving an assignment to setup a maill server using debian as os and postfix , dovecot so that user can setup there mail client (thurderbird and outlook) to send mail on LAN to each other, please i can i get it done please

Then go to postfix.org and dovecot.org & RTFM

Both have really good documentation.

Here are two howto guides that I think are pretty decent: http://flurdy.com/docs/postfix/ and http://johnny.chadda.se/article/postfix-howto/

Now, being a homework assignment, I can guarantee you that it is MUCH more important for you to understand several key fundamental concepts about what you are doing than it is for you to actually get a working mail server. For example:
1 - most people think of an email server, when in fact SMTP and POP/IMAP are completely different functions and entirely different server processes
2 - the critical role that DNS plays in email and MX records
3 - Mailbox and Maildir formats
4 - virtual transport and aliases.
5 - UBE (unsolicited bulk email, aka SPAM) protection
6 - Daemon processes under linux and the critical role that file and folder permissions play
7 - database integration
8 - SASL authentication

In addition to above - you noted outlook as your possible mail client (mail user agent).
Remember outlook is not simple mail client. It has also advanced functionality (shared calendars, shared address-books ...) which has nothing to do with mail servers. For you this may be clear, but the end-users do not really understand the difference.