Can someone tell me when to use which one "Tu/Su"
Both Tu and Su mean "your" So how do you know when to use which one?
There are so many different ways to say "you/your." Tu means your and tú means you. Su can mean you (formal) he/she/its. Depending on how you conjugate the verbs in your sentence to either use the second person single or the third person single will determine if you are going to have to use tu or su. Tu is informal, as in when talking to friends or those you know very well, where su is something you would use for parents, teachers, strangers, police officers, etc. Anyone with whom you would address respectfully.
This link should be helpful in discovering which form of the word "your" to use and when. - http://www.spanishdict.com/answers/100031/possessive-adjectives
Both "Tu" and "Su" can and have been used interchangibly.