I'll is the short form of I will or I shall. You use it when you talk about a future action, a promise, or a decision: “I'll help.” In speaking, I'll is the normal form; saying “I will” every time can sound slow, or extra strong when you stress will (“I will do it”). I shall is older and more British, and many speakers now use I will for all persons; I'll covers both. The apostrophe shows that letters have been dropped. I'll is perfect for conversation, messages, and stories. In very formal school essays or official letters, some teachers prefer I will. Though actually, I'll is not past tense, and it is not the word ill (sick).