"Here it comes" is used when something is coming. That's pretty much it. There is nothing more to it. Maybe you are throwing something to your friend and just to warn him you can say. "Hey, here it comes.". Maybe be you just ate and tell your friend, "Oh I am gonna burp." Right before you burp, you can say "Here it comes." and then burp. I know it is a weird example.
Now, "Here we go." can be used in different ways. Maybe you are about to get on stage and as final words, you can say "Here we go." In this case, it literally means you are going (on stage and perform)
Your parents might be about to give you a lecture, and in an irritate and sarcastic tone you can say, "Here we go (again). " Really doesn't have any specific meaning. Just depends on context.