"I" should be used when you talk about yourself or want something. For example, when you say "I went to the beach last summer", you are talking about yourself. Another example is when you say "I want a cookie", you want something.
"Me" should be used when you want someone to interact with you or express their opinion about you. For example, when you say "Help me!" you want someone to make an interaction with you. Another example is when you say "Do you think that these jeans would look good on me", you want someone to express their opinion about you.
