My laptop turns off by itself.?

My laptop is a dell laptop. It overheats and turns off. but now after not even 5minutes of usage it turns off by itself. My sister says its because I don' put it on sleep or hibernate ,& that i just close it. Could that be it? i put it on sleep but still turns off. What can I do without having someone like best buy fix it? it could cost money and i don't have that much money to fix it. Do i reprogram it or what? Please help, Thanks -Angie