Grappa is typically served in a small glass called a "Cappelletti." You are supposed to drink it in one shot, but I recommend sipping it if you want to avoid being knocked on your ass.
It's fairly common to see grappa served with a piece of fruit, typically a slice of lemon or orange. This is supposed to help cut the bitterness of the grappa.
If you're on the fence, grappa is best avoided altogether. Again, I think it's awful, but if you need to try it, sip it slowly and chase it with a piece of fruit.