bword

Hey everyone new to fixing up bikes here and had a few questions. This mountain bike was given to me needing some work. Its a Diamondback trail xc and the tires are beat,the seat,and brake pads all need replacement which is cake for me. The biggest issue is rear shifter. I do not know the bike terminology so im looking for help in what I should be hunting down for parts. The rear shifter was bent and broken off the frame. I believe all Id need is the rear shifter mechanicism and the bracket that it connects to which bolts to the frame, I would get a chain too just for good measure. Does anyone have a good website for replacement parts and what I would need??

thanks everyone