26" Mongoose Excursion

[ntomita]

Hi, All!

I recently got a broken bike from one of my friends, and want to fix it as a fun project.
Seemingly its chain is off from the rail and completely distorted like a helix, so it needs to be replaced. Also, rear derailleur is shattered into two parts, each has a gear, not sure if it can be repaired as I'm a total newbie, so I decided to buy a new part for it as well.

However, I don't know which parts should I buy. I checked official documents for maintenance but it doesn't describe the specifications of parts. Does anybody have ideas which parts to buy? I'm happy with 3rd party parts for cheaper cost or used parts from ebay. Any input is helpful! Thanks in advance!

Here's my bike:
https://www.dropbox.com/s/hdf6spnm8k9bk3j/2017-08-23%2010.48.59.jpg?dl=0
https://www.dropbox.com/s/vgrz3i8zdz5b6oy/2017-08-23%2010.49.11.jpg?dl=0
https://www.dropbox.com/s/hfl0fb8gy1qy0uj/2017-08-23%2010.50.02.jpg?dl=0
https://www.dropbox.com/s/sqotby6zqsvywys/2017-08-23%2010.50.28.jpg?dl=0

Also, this is the same model:
https://www.walmart.com/ip/26-Mongoose-Excursion-Ladies-Mountain-Bike/48520137