when do I use mi or me

Mi (without the accent) is "my". Mi libro. My book. If books is plural change it to "mis libros.

Mí (with the accent is "me") it is used like this. Esta enchilada es para mí. This enchilada is for me.

Me also means "me" but it is used as a direct or indirect object.

Pablo me vio en la calle. Pablo saw me in the street. (direct object)

Pablo me dio un regalo. Pablo gave me (to me) a present. (indirect object)

I have simplified this as much as possible. It is still better to learn grammatical terms as my wonderful friend Bosquederoble has suggested.

The point of grammar regarding "gustar" is another matter. On this site we have great teachings on various points of grammar. You will find the links at the bottom of the main webpage.

I do not know if you meant mi or mí- but the accent makes a difference.

Me is the pronoun when you are using it as a direct objectan indirect objector a pronominal/reflexive object.

Mí is used when it is the object of a preposition.

Mi is a possessive adjectiveand can become mis if the noun it is affecting is plural.

A mí me gusta manejar mi coche.

Mí- because it follows a preposition (“a”)- to mein my case.

Me- indirect object of gustar. It is pleasing to me.

Mi- it is my car.

This is all opposed to "yo" which is the subject pronoun.

http://users.ipfw.edu/jehle/courses/PRONOUNS.HTM

If you do not understand these grammar termsI would suggest learning them.