Refer: http://superuser.com/questions/144453/virtualbox-guest-os-accessing-local-server-on-host-os
Enable ONE network adapter. Set it to NAT and start your machine. Don't need to configure anything.
On guest machine's browser type http://10.0.2.2 which will show you the same content as "localhost" in the host OS.
10.0.2.2 is the "default gateway" for NAT connections and your guest OS' IP is 10.0.2.15
Of course you can set a "HOST-ONLY" or an "Internal Network" which will allow you another advanced configurations. I recommend to stay with NAT.