Samba is one option as Invisix said. The easiest option for simple file transfer is Amiga Explorer from Cloanto.
http://www.amigaforever.com/ae/It works great with tcp/ip(with serial nullmodem too, but it's slowslowslow).
-Go online with genesis
-Start Aexplorer on the amiga(it should autodetect tcpip connection)
-Start Aexplorer on the pc and then a window opens on the pc that gives you access to all drives on the amiga.
