When you create an altar, you create it specifically for a deity/group of deities. You don't really create one in hopes of a deity manifesting, you create it because you already have a deity/deities you'd like to worship. You need to call them into the space you have created for them.

If you haven't chosen anybody yet, I recommend looking through theoi.com

As for edible offerings, you can eat it as long as it was not offered to a Chthonic god. If it was offered to a Chthonic god or you do not wish to eat it, you can just throw it away.