normally the closest word to the verb is emphasized. in your example tuzlu becomes adjective and the object becomes 'tuzlu makarna'. so the translation becomes something like i am eating salty pasta.
by the way you can always change the tone of the sentence in order to change the emphasis just like in english. so you can say ben TUZLU makarna yiyorum. that would emphasize the fact that it's salty. however ben makarnayı tuzlu yiyorum is better when writing because it assures that the focus is on the word tuzlu for the reader.