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.