Low computer space

I bought this laptop last week.  It worked fine, except it kept going to sleep every five minutes, and I was unable to change that.  Then a notice kept appearing that my computer space was low.  I checked my pc, and it says I have 851 Megabytes free of 99.8 GB.  That seems excessive, I don't know what is taking up so much room.  Today, I could not open applications without them crashing.  And Microsoft Edge would not stay open.  This is a brand new computer, and I don not understand why this is happening.