Chain keeps slipping I was pedaling my bike, the bike has never been used and sat in the garage through winter. While I was pedaling I switched gears and adjusted the pressure I needed to pedal (don't know what this is called) and the chain slipped and when I pedal now it makes  like a scratching noise and the chain doesn't move. I've put the chain back on multiple times but if I pedal a few  times it happens again I'm 17 and know nothing about bikes. Any help?
I think your derailleur needs adjustment.  Watch this video 
                
            

and if you think you can follow those steps, try it.  Otherwise bring your bike to a bike shop and let them do it.