SQL Server Dynamic Port change

I have a named instance which has been assigned default port 1433. I want to change this port as 1433 is default port. How do i pick up a new port number? can that be any number ? I know how to change it but what i need is what port number do i need over here? Do i need to change any fire wall rules. I have never changed any ports while i did installation, how does those ports get assigned and how do they work?