Will not launch

For some reason. I am unable to launch my MacBook. I used to have an unapproved caller message saying: Securityagent may only be involked by AppleSoftware. It wouldn't let me get passed that. After I restarted it, messagewas gone. But now, it doesn't say that. A new problem came up. When ever I launch my computer, it would load half way, show a huge message numbers and letters, restart, and do the same thing in an infinate loop. On its own. When it finally launched, I was unable to open any apps that I downloaded, just the apps that my macbook came with. It says I have to restart my computer. So i did and now I can't launch my macbook anymore because it would load halfway, write a huge message, restart, and do that infinate loop until i have to force close my computer. Please reply soon because I need to use my computer for School/Homework purposes.