Use "this shit" to refer to something that you don't like:
Ew. What is this shit?
I don't have time for this shit!
"Shit" is a curse word, so there are many situations in which it's not polite to say it. You shouldn't say "shit":
- around young children
- around strangers who you want to impress
- to people who you are trying to impress, like your boyfriend or girlfriend's family
- in church
When you do use the phrase "this shit", it sounds angry and tough.