Use is when the subject is already there: The coffee is cold. If nothing comes before the verb, supply it: It is cold in here.
When you're announcing something exists, that's there is: There is some cake left.
Weather and time use it is naturally: It is snowing. It fills the subject slot.
When you're saying something about a thing you've already named
You've already named the thing: The soup is too salty. Since a subject comes first, is attaches straight to it and no extra word is needed.
Same pattern: The meeting is at three. Your wording tells the listener you're commenting on something already on the table.
No subject yet? Supply one. Is snowing isn't a sentence; say It is snowing. The dummy it fills the empty subject slot.
Judgments work the same way: Is important to arrive early sounds incomplete, so write It is important to arrive early. The real subject, arriving early, sits later in the clause.
Weather, time, and distance follow the pattern: It is five o'clock, It is ten miles to town. There's nothing specific for it to point at; it just fills the slot.
One boundary: when you're announcing that something exists, English wants there is, not it is. Say There is a woman waiting outside, not It is a woman waiting outside.
When something exists but no one has mentioned it yet
Something new has come up in the conversation. If you're announcing that a thing exists, English usually wants there is, not it is. It is a woman waiting outside sounds off; say There is a woman waiting outside. There fills the subject slot when no real subject yet exists, because the woman is new to the conversation.
Answering a question can flip this. If someone asks What is left?, the direct reply Some cake is left puts the answer up front. For everyday announcing, though, There is some cake left is the more idiomatic wording.
Don't overdo it. In formal writing, stacking there is sentences can sound vague and wordy, so some editors rephrase with a real subject. Both versions are grammatical; the choice depends on emphasis and register.