can any one tell me about this

how to connect 4 offices through VPN
hello My office have 3 branches and we want these branches to connect our head office, our requirement is to share the data between these 4 offices like Excel, word and oracle is our data base server, we just want to share the data between these 4 offices with VPN. Can any one tell me what is the minimum requirement and procedure for this network setup and what type of routers we can use, and what about the internet leaseline or 1Mb ADSL line is enough for this setup