What's the difference between もちろん＆もちろんです?

[YozakuraHime]

"です" makes it polite. I would stick to polite language until you have more experience in the language, so try to use です (or ます forms for verbs) when you can.

[BJCUAl]

Just echoing YozakuraHime's answer. Similar to the difference between 'Of course' and 'Of course it is'. No difference in meaning or nuance. Just a grammatical difference. I would also agree to stick to 丁寧語 (ていねいご - polite language）by using です or ます by default.