That depends on your relationship :)
Does the negated accusative not form the genitive case in Ukrainian like it does in Polish or is this in the genitive case? to me it looks funny just cause its basically the same as in polish but using the "wrong case" if it were in polish (it would be "nie jem miodu" in polish)
Neither of them means "to eat" (infinitive)
As in English, you cannot say "I is" or "you am" or "he are" -- you have to use the right verb form that fits the subject. "I" always goes with "am", for example.
And so in Ukrainian, їм is the verb form for "(I) eat" and їсте is the verb form for "(you, plural) eat".
So if the subject is "I" who is eating, you have to use the form їм.