Need to delete all files...

i was "lent" this computer my my school district 4 years ago... though i never had to sign anything for it, and the person that gave it to me made it seem as if it was a gift or something. id just been through a lot of crap caused by a teacher who had been lying and nearly got sent to prison, so i figured it was a [please don't sue us] type of gift... especially how it was given to me...

i had to go meet a teacher at the public library, she gave me a used macbook in a box to do schoolwork on, once i was done i tried to give it back to her (thinking it was borrowed just to be used there with her) and she said "no, that's yours now. take it with you."

so i was never sure if it was borrowed or to be kept. anyway... i have replaced the screen, battery, and fan on it, and its a 6 year old macbook. :/

so basically, now they want it back. im an artist, i do a lot of photography, painting, ect. all of it is saved to my computer... and due to the circumstances of what that teacher who caused all the trouble in the first place was accusing me of, some of the more ah... dark content of my computer could be a problem. im a bit paranoid they'd try to use it against me. nothing too much, but i do tend to paint some messed up things like weird monsters, skeletons, gas masks... and most of my photography has a gritty eerie feel to it and there's a few with fruit with knives stuck in it with fake blood pouring out (its supposed to be funny, but they wont see it like that.) that and over the course of 4 years i may have googled a few things im not proud of and downloaded things that im not sure are legal (i don't torrent, but... i don't check carefully for copyrights before i download something either. so basically if i know i shouldn't download something i don't but ignorance is bliss i guess.)

anyway, i need to clear this stuff out, because they would use anything against me they can... its nothing too bad, pretty much what quite a lot of people have on their computer, but i need it gone. ill back up what i want to keep of course.

ok enough backstory, i just figured want to know the reason, lest your minds wander

so, any advice on how to completely clear out all files? i mean, keeping the apps is fine of course, just my own files. ive heard anything stuck in the trash can be recovered...

>>>>>>>>TL;DR version: i need to clear out my computer's files completely, i will back it up first, but how should i clear it with no traces?

You really didn't need to post the history and background. To wipe the machine clear of everything you will need the recovery or install media; whichever OS X disk that came with it. Boot the machine with the disk, and then select Disk Utility. Use Disk Utility to erase the drive (you can use the security wipe feature if you like) and then reinstall OS X.

i don't really have the install disks... they never gave me those.

are there any other ways of doing it?

You can get yourself snow leopard from Apple or Apple reseller for US$29, and use that to wipe your HD as Chscag advised.