Need advise on Security , SSH , security policies

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http://www.linuxquestions.org – We have a 30 Computer Network , one small business Server and 29 Workstations , we have 10 Servers hosted in a Data Center (running on Red Hat Enterprise Servers) Communication normally take place between SBS server and 29 clients (internal) , between 29 Clients and 10 Data Center Servers through SSH , Between external Clients accessing 10 Servers at Data Center , Between External Clients accessing SBS server and 29 Workstations we are also running Apache Web Server , QMail , Mysql Server on our Data Center Servers i need advise that what will be the best practice to secure all these communications between internal/external clients and Data Center Servers , between external clients and SBS server/internal clients , within our office network currently all of our communication within network is not secure we access Red Hat servers in data center through simple ssh external clients access data centers and our office through remote logins and ssh so how can security be implemented internally as well as remotely (accessing data center servers & external clients) please be as brief as possible , (HowTos)