I just wrote a massive paragraph a couple of hours ago trying to explain the difference, got distracted by something shiny on the floor, and then accidentally deleted it. Here we go again.
The differences are pretty subtle and quite hard to explain when you haven't studied English grammar and have just grown up knowing when to use both of them like most native English speakers. However, I'll give it a go.
"It" generally refers to things like time, distance, weather, and the general environment or feeling of a place. Here is a conversation illustrating some common examples of "it is" being used in these ways (Note: "it's" is generally more common than "it is", and I'm sure you're aware that you can also say "there's" instead of "there is" seeing as you seem proficient at English):
"England makes me sad; it is always raining here." (It is actually raining right now over here)
"It isn't that far to travel to the rest of the Europe, so why don't you just go and live there you filthy nation traitor?"
"I don't have time to do anything but grovel pathetically and hate my own people. It's too late to go there now and I'm just too much of a bloody cretin to do anything with my life."
"You really are a cretin, aren't you? Yes, that's right; dance for me, cretin! Now begone... It's ruining my good mood. Begone I say!"
Like Mizal said, "It" also refers to objects that have been specified already, marking them as the subject of the sentence. For example:
"The ball rolls towards you. It is red, shiny, and smooth." Here, "there is" wouldn't work because the word "there" tends to specify the existence of something, whereas "it" is used to describe the object. This is explained more below.
"There" is used to express things like position, events, and existence. Here are some examples:
"There's a party tonight! Do you want to go?"
"With you? Ha! There's no chance I would be let in with someone like you."
"There is a ball by the tree."
"What? Where the hell did that come from?"
"It is just an example, okay?!"
Getting a bit off topic towards the end, but I hope you get the idea! One more tricky example of "it is" would be the following:
"You burst into the room. It is filled with a range of strange objects." Here, you could say "...there are a range of strange objects." and it would make perfect sense as well. "It is" in this case, I think, describes the room participating in the verb of being 'filled', so even though it does describe existence (of the objects), I think it's just a secondary consequence of the description of the room as being 'filled'.
I hope this helps! There may be a few other examples where there are differences between the two that I might have missed. That's partially why English is considered one of the most difficult languages; we have so many exceptions to rules! Good luck with your writing and, just out of curiosity, what is your first language?