I've learnt from a guy certain substitutes for certain uses of the f-word. However, as you must realise, the f-word is extremely versatile; no other word even comes close to its versatility. Thus, many words would have to be used just to substitute this word. As the use of this word is apparently frowned upon, it might be worthwhile to find certain substitutes.

Original-O Substitute-S

O: "You're f**ed."
S: "You've really messed up bad."

O: "F***!"
S: "Goodness gracious me I've/you've/it's really messed up."

O: "F*** off."
S: "Shoo."

O: "F*** you!"
S: "You're a real mess."