I dont know how to make a new save file and i cant find any info on the topic. I need this because i cant to full-game runs because in the rules it says that it has to be done on a new save file.

In the main LEGO Racers folder, there is a folder called 'Save'. Click on that and you'll get folders from 0-14. The game always uses folder 0 for the save file. If you want to save the save file (one with all the cars and people unlocked), move the save file to a different number. You can leave Folder 0 empty; the game will make a new save file upon launching the game.