How do you say goodbye my friend in Japanese?

Formally 'goodbye' would be 'sayonara' and 'take care' would be 'itterasshai',or 'okiotsukete kudasai'; when speaking polite or formal language a combination of these phrases is hardly used.

In colloquial language for 'goodbye' you could use 'bai bai' (girlish or teenage use), 'ja ne', 'mata ne', and for take care you can use 'genki de ne' or 'kiotsukete ne', and each of those phrases in informal speech can be used together.