Magick isn't anything all by itself. At times when an action happens within the universe it effects the action by a small factor. But if someone uses magick knowing it will do a certain action, it becomes more powerful in that course. This is why defensive magick is a must known to protect yourself from other spell casters and yourself.
The first type of curse is a simple threat. It is where the person tells you they placed a curse, hex, or even demon on you (Not all demons are bad, but that doesn't mean they can't use one). In most cases it is just their bark and no bite, but even so people that get threaten still believe they cast one on them. This is a simple law called Mind Over Matter. If you believe something so much, it will happen without it happening. Basically you believe you are cursed by the person so much that you start to feel sick or whatever you believe the curse is. This can be combated very simple and will take some guts for people to do. Simply laugh in the person's face when they tell you they will curse you. What this does is two things, it makes your mind go at ease which reduces you from hurting yourself thinking you are cursed and it sets the person with the threat back and worried. Even if that person is angry, Mind Over Matter affects them too but in a way that makes them believe they cannot really do anything to that person.
Another type is when a person really does want to curse you. Normally they will not tell you this because they want it to work without you stopping it. Since this is the online world I'll split reality from it. In the online world, your energy does not follow your text. Your username has no connect to you as a person (unless it is your real name). So really you cannot go on this site, pick some random member, and curse them with the information they lay out. The reason is because curses and spells need a few things before they can work.
Spells and curses need to know who they are going after. This is normally done with their first and last name, but can be done with an item they hold dear to or cursing an item and placing it in their house. This can also be done if they have your DNA (nails, hair, blood, ect) or picture. So as long as you don't give them a chance to get any of those you really cannot be cursed by someone. That really does make cursing someone online really hard (Unless they use mind over matter).
And lastly, you cursing yourself. I am not saying you are going to search for a curse to place on yourself, I am saying a spell you know little about is going to backfire on you. Backfiring spells are bad if you do not fully understand the spell at hand. This could be as simple as a luck spell and as tough as a rite of summoning. The only way to reduce the chances of a backfire you must study the spell and practice it without really casting it. You should also know how to cast a circle or a stronger barrier. Sadly there really isn't a way to stop it completely because we are all human beings. There is always the human factor.