I have seen that は acts as a sort of negation (or contrast) particle, although I'm not sure how it works, for example:
1) わたし は ともだち が いません
2) わたし の ともだち が いません
Is 1) the only correct sentence? Why? And what's the rule for using は instead of other particles? I have tried searching a bit but I still can't seem to understand this.
Another example would be
1) さかな は たべません
2) さかな を たべません
Is 2) incorrect? Why? Are there any other cases where I should be aware of this?