If you already have a running mongod instance you first need restart it with --replSet
$ mongod --fork --rest --replSet mySetName --port 27017 --dbpath ~/Projects/db/data/mongo/r0 --logpath ~/Library/Logs/mongod-r0.log
Initialize the node via the mongo client
$ mongo
> rs.initiate()
{
"info" : "Config now saved locally. Should come online in about a minute.",
"ok" : 1
}
This node is our Primary Node where read/write is done initially.
> db.messages.insert({City: "Tokyo"})
> db.messages.find()