What kind of rules or limits need to be placed on magic? That will vary from world to world. Maybe an individual can only use one type of elemental magic; like fire, water, earth, or air, and nothing else. Maybe the use of magic costs the user something; like strength, or life.
Whatever limits you place on magic, you must be sure to apply them evenly and equally to everyone. Don't have your hero collaspe from exhaustion after casting a very powerful spell, just to have your villian cast two equally powerful spells one right after the other.
Some questions you can ask to help make your rules are:
1) What is the cost of using magic? (Strength, youth, time, energy?)
2) What is needed to cast a spell? (Talismans, ingridients, potions, powders?)
3) How is a spell cast? (Spoken words, thought, hand motions, with a wand?)
4) Is magic legal? (In one of my stories it starts out illegal and the protagonist must keep her abilities secret to stay alive.)
5) Who can do magic? (Everybody, just a few special people, family lineages?)
These are just a few of the questions you can ask yourself when creating a world with magic. Try and come up with a few of your own and see what kind of magical system you can create.