I is always (or should be) in the subjective case, me in the objective case, and myself in the dative case. Example:
I answered the question.
In the preceding sentence, I is the subject. In the sentence, "She wants me," me is the object of wants. In the sentence, "I did it myself, myself is a reflexive pronoun referring to I. Never use I after a preposition, unless, as in the previous sentence, you are referring to the word itself.