Demons are mainly going to harm others, if their around. So to protect yourself the main things that you can use are:
Salt- ordinary table salt... you will need quite a lot.
Holy water- Water that has been blessed and prayed over.
Either stand in a circle of salt, or put a line of salt around every window and door in the room/building.
Demons cannot pass through salt, but make sure there isn't a fan or anything that could potentially be used to break the salt circle/line.
With the holy water, you can just splash it over the demon.
Holy water is the safest protection against demons if they are possessing someone.
If, for any reason you need to summon a demon, this is usually done at the cross- roads. A box with your picture and 'some other things- can change' is buried just under the surface of the road.
To get rid of a demon, the best way is to perform an exorcism (especially if the demon is inside someone). Or to use a silver bullet or knife- NOT ON A HUMAN!!!! So only use the silver if the demon ISN'T possessing anyone. Rocksalt bullets are also good.