Drilling 3D-printed plastic
Have to get this out of the way... Can you just print the hole into the part instead of drilling? That's usually best, when possible. 

If you need to drill, you'll definitely want to print an undersized hole and then ream it out with the drill. This both provides alignment for the drill and adds extra plastic around the region you'll be weakening. If you don't know where the hole needs to go in advance, fill the model with a hex pattern of tiny holes in the general region of the future drilling. This is an easy way to strengthen the area with more plastic without making the entire part solid. (Using a slicer that allows localized print settings will also work.)

The hole should be aligned more or less vertical or "across the grain" (+/-45 degrees aligned with the Z axis) to ensure there are nice, strong hoops of plastic around the hole to keep it from splitting. Drilling into a printed part (particularly PLA) from the side / parallel to the XY plane is highly likely to split it. It may even be ok at first but crack later under load or due to creep. 

Regular wood drill bits work ok, but you need to go slow and take nibbles, leaving the plastic time to cool so it doesn't melt and stick to the bit. (Coolant is an option.) Use a pilot hole, piloted bit, or step drill to remove less plastic per pass. A really sharp bit will perform dramatically better than a dull bit. 

Put something hard and drillable like wood behind the plastic part to help avoid breakage and bit-jamming when you break through the opposite side.