"Shit" creates special problems for Spanish speakers of English for several reasons. First, it's not used as commonly in English as it is in Spanish; maybe it's because I live in Madrid, but I hear people here saying it a lot more frequently than I did back home...and, trust me, people where I come from swear a lot.)
Here are some basic guidelines that you can use when you're using this word:
shit - "a swear word that many people find offensive, used to show that you are angry or annoyed (OALD)". English speakers do NOT automatically use it to describe everything that works badly (Better: "This is useless" or "This is a useless piece of junk"), tastes bad ("This is disgusting/awful/horrible"), or as a countable noun. Similarly, if your boss lands you with a stressful, last-minute task, that is not called "a brown" in English.
There are various variations of the word that, when combined with the name of an animal, gives another idea of "extreme emotion", but be careful because they are used in very specific ways:
bullshit - a swear word that many people might find offensive, used to show that the person thinks the information is highly incorrect or deceitful. ("He told you that he's going on holiday with his grandmother? Bullshit!")
apeshit - (esp. to go apeshit) - to become extremely angry, especially when you receive bad or shocking news.
batshit - (esp. batshit crazy) - extremely irrational or crazy. (The politician may have some good ideas about the economy, but I'd never vote for her; she's batshit crazy.)
(One note: don't try looking up definitions for these words in either Wikipedia - which will direct you to "anger", instead; or UrbanDictionary, which has no filter process that eliminates useless or incorrect definitons.)