First step I can think of is decide which technology direction you want to take(whether you want to do it using J2EE or .net etc etc)
Then the next step I would consider is the domain of the system and put a process in place for the development of the enterprise system(may be RUP or XP etc etc).
Then if the choice is J2EE, hire correct set of resources that can achieve a good system using the selected process. Also concentrate on learning the crucial technologies(like JSP,EJB, Hybernate, Oracle, JDBC, JNDI, JMS etc) and select the right set of those for the particular system.
Thanks Kishore, Sorry may be i wasnt clear in what i asked, i wanted to know from which end of the architecture would be a good start for enterprise Application, i understand that everyone would have there own way of starting but which one is the mose preffered way?