HELP! mac computer issues

I hope I am posting this in the right area!

I have a mac computer and desperately need help cleaning out old files, organizing and etc. There are things on there that I don't even know what it is or if it's important that I keep it. It's running slow and I am having to reboot it once, sometimes twice a day. I don't feel comfortable handing it over to Apple. Please respond only if you are a mac computer geek. I love geeks by the way.

Thanks,
Juliette
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