we have a small network ,one DC (2003r2) and an exchange server 2003 on 2003R2 that is not connected to wan,use for local network and all the mail store on clients system (pst). there are more then hundred user now i have got new server with windows server 2012 and i have to install exchange server 2013 . what should i do.

There are plenty of things that you can do but since you are using just for internal mail flow purpose you don't have to configure mail flow or external URLs as Exchange is being used in internal network. You can configure Internal URLs on it though. You can check below list of Post-Installation Tasks. Exchange 2013 Post-Installation Tasks

There are plenty of things that you can do but since you are using just for internal mail flow purpose you don't have to configure mail flow or external URLs as Exchange is being used in internal network. You can configure Internal URLs on it though. You can check below list of Post-Installation Tasks. Exchange 2013 Post-Installation Tasks

Thanks for your reply but i would like to know that should i remove the ex2003 and the install 2013 as migration is not supported and all users have their mail in his pst's.

Oh yeh, missed that part, Exchange 2003 to Exchange 2013 direct migration is not supported and also you need to remove all Exchange 2003 servers before you install Exchange 2013 so you would need to go to Exchange 2010 first. If you uninstall Exchange 2003 and Install Exchange 2013 then when you create mailbox for all the users, if it creates new legacyExchangeDN then users won't be able to reply to the mails what they have in PSTs so I would go for Exchange 2003 -> 2007/2010 -> 2013 path...

Oh yeh, missed that part, Exchange 2003 to Exchange 2013 direct migration is not supported and also you need to remove all Exchange 2003 servers before you install Exchange 2013 so you would need to go to Exchange 2010 first. If you uninstall Exchange 2003 and Install Exchange 2013 then when you create mailbox for all the users, if it creates new legacyExchangeDN then users won't be able to reply to the mails what they have in PSTs so I would go for Exchange 2003 -> 2007/2010 -> 2013 path...