Open System Preferences (Apple -> System Preferences) and click Sharing.
Select Internet Sharing from the list of Services.
From the Drop down menu, select the interface which is connected to the internet and from the list below, select the interface which you want to share with the other devices.
The final step is to create an adhoc network that the other computers/devices can connect to.
Click on Airport Options.
OS X will then ask to type in a name for your network.
I’d strongly suggest that you select the 'Enable Encryption' and provide a password for the network.
Click Ok and OS X will enable Internet Sharing from the Ethernet to the Airport Interface.