The following incantation is a general defence against demons and evil spirits. Not all demons will be defeated with this, but even when the spell is not sufficient to actually overcome the spirit, it can still hurt them enough to keep them at bay.
If spirits threaten me in this place,
Fight Water by Water and Fire by Fire,
Banish their souls into nothingness,
and remove their powers until the last trace.
Let these evil beings flee,
Through time and space.
For more serious cases or psychic attacks, check out this more advanced protection spell.
Successfully performing a spell is an art that comes about with practice. The better you are at working with magickal energy, the better your results will be. The Essence of Magick is a guide which teaches you how to cast spells that work.