You decide if you use it or not. The information about the substantive is already included on the verb conjugation, so you can say, e.g.: "Me llamo Juan" or "Yo me llamo Juan". Preserving the pronoun "yo" only makes emphasis on who is speaking (who is the sentence subject/substantive), which can be useful in some circumstances (to place intentional emphasis), e.g. : Q: "¿Tú eres Juan?" (Are you Juan?) A: "No. Yo me llamo Pedro" (No. My name is Pedro) Yo no comparto tu opinión ("I don't share your opinion") Yo hice el trabajo. Él no. Yo merezco el aumento ("I completed the work. He didn't. I deserve the raise")

记得李某某中教曾告诉我们，因为人称后面的soy神马的那些已经能确认此人是谁了，所以可以省去人称。(没记错是这样的)

Using "yo" sounds very repetitive to me. In earlier classes they have you use it so that you can conjugate verbs easier. By saying yo it reminds you to add the -o ending, but after a while, you start to slowly use the yo/tu/ud/ellos less. I tend to never use them in speech and use them sometimes when I am writing in Spanish.

No you don't have to say Yo in front, actually you should simply say " me llamo..." Without you because it just sounds more authentic. Another natural way you can introduce yourself is using yo is " yo soy...." This pattern is very common in everyday spoken Spanish and even Portuguese