Dell Inspiron 15 series 5000 won't start up

Hello there! My dell inspiron is not starting correctly, it happened weeks ago and I don't know how to fix it. When I open my laptop it either want me to restart it (where nothing will change) or go to advanced setting, I want to use and try all things but it says there that only administrators can do it, which I can not do because my sister was the last one who used it, how can i change my account from guest to administrator? Please help me