As mentioned in other comments, the start of the game needs some work. Either a count down or the game should start paused or slowed and then resume at full speed once the first jump input is triggered.
More importantly than all that tho, you need change the input events you're using to register a jump. You are currently using either "click" or "mouseUp" events, and you should be using "mouseDown" and "keyDown" events. Those "down" events will trigger the frame the input is registered, where as the "up" events trigger the frame the input is released. That's why everyone is having a hard time with the jumps. Currently the jumps only trigger when you release the left mouse button, not when you first press it like expected.
