How do connect 2 offices ???

My problem is: I have offices on different countries… people on these offices wants to share information and software… we have different domains… my idea was to link them using the public network (aka internet) and stablish a trust relationship between these domains… but… how can i do it safely and in a not so expensive way?
Like… may i use a VPN? How can i do it? Im open do suggestions… 🙂