You should never eat raw wild acorns straight from the tree because they contain a significant amount of tannins, which are toxic to humans in high quantities.
If you're going to grind them up into acorn flour, soak them in cold water, and if you're going to use them whole or in larger chunks, use boiling water.
In addition to grinding them for flour, you can roast them for 15-20 minutes at 375 degrees Fahrenheit. Some people like to roast, grind, and brew them like coffee beans.