Take for example the word they use for bread (パン(Can't read it?)[Pan]). If you were to assume the word was from English and thus pronounced it with an English accent, you'll undoubtedly draw stranger than usual stares, since other English speakers in the crowd might conjure up images of a cooking pan, or even a panning shot(both also what the word can refer to), instead of the bread you intended. Think about if someone pronounced the word Bologna as Bah-lone-yah (the approximate original Italian pronounciation) instead of how everyone who speaks English pronounces it (Buh-low-nee) if you want an idea of just how off putting that is. For anyone curious, パン's root in this context is actually Portuguese(pão), which is also where word for raincoat (カッパ[kappa]) oiginates from(capa).
So remember, when speaking with Japanese speaking friends of things outside their country, keep Katakana in mind and pronounce it through their accent-especially with mora in mind. This will also help you in getting what you want from Japanese establishments, even if how it's pronounced in Japanese seems nowhere near close to how it sounds in English-Vodka being pronounced as ヲッカ(wokka), for example. More on what the small つ(tsu) signifies in speech, as well as how to pronounce foreign words in through Japanese phonetics later.
Special note: steer well clear of outdated slang in your spoken vocab. You know how you look in pity at an old person that says, "Radical dude!" in an attempt to be with the kids? Unless you're trying to be ironic-which isn't impossible, but harder to do in a foreign language-that's how you'll look when you use super old slang, so be sure to keep your knowledge of it up to date by asking around before you pop in out in public. Also keep in mind slang isn't just limited to what everyone you know uses. You can also invent some of your own, the more you learn about the language, which I will cover in further detail later on.
There are also instances where what the word means in Japan takes a 180 from it's mother language's meaning. Two examples of it are the words smart-which is used to say someone's suave(slim, stylish, cool, handsome, and well dressed being encompassed in the sentiment of Smart)- and Mansion-which refers to a condo or penthouse. Bear in mind that these are both slang uses, so, because this bears repeating, be sure to ask friends to see if these are still relevant before employing them in your everyday Japanese
In modern times, this phemonenon has even inspired, and continues to inspire the use of Japanese and other languages to invent brand new words, such as with the terms Light Novel (a shorter form novel-think Novella, or The Green Mile series of novels-peppered with illustrations) or バ―コ―ド