Hi im trying to start speedrunning minecraft. Today I downloaded LiveSplit.
But I dont know how to open it while my Minecraft (java) is open. Any advices? Or another timer tip?

If you want to show the Livesplit timer on top of the game screen, while the game is in full-screen mode; you should open both the game and Livesplit window as administrator.
If you are asking about how to start/split the timer while the game is in focus; you should enable "global hotkeys" in Livesplit settings.