There a few meanings of "here we go" 1) We are about to start our rollercoaster ride; the ship/plane is about to leave 2) Some people use "here you go", when pushing a child on a swing 3) It may be said when the hostess is about to serve a meal; Here we go, dinner's ready. this is NOT the same as "here we go again" which is an expression of impatience (Oh no, not again)

"This is starting" would be a rough approximation, it indicates that you know about what is going to happen e.g: At the top of a rollercoaster - "Here we go!" (we are about to drop rapidly) Somebody you don't like walks into the room, you say to your friend - "Uh oh, here we go" It can also be used (I think this is might just be a British thing) when you find something - e.g [Rummaging through a box], "Ah, here we go! I knew it was in here somewhere!"