This doesn't seem like a problem centered on first-person perspective, so although I normally write in third person, I'll give you a bit of tips.
1. Practice varying sentence structure. If you always begin a sentence with "it," then your problem is probably in that you'll need to start playing with how you start your sentences. Try using dependent clauses or introduction words.
2. If the problem is the word itself, try describing what the object is and using it as a name. If the object is a furry rodent, call it a furry rodent from time to time. If it's a creature, then you can call it a creature.
3. The answer that does center on first-person perspective: Know your character's voice. The point of first person is that the story is told by a particular character in the story. Thoroughly understanding how that person speaks (their tendencies, their use of language, et cetera) will help you figure out how they'd refer to things. If they would call everything an it, then you can leave it alone and get away with it. If they wouldn't, try to think of what they would call it instead.
Hope that helps a bit.