Tires on bike won't move and derailleur issue

[Nlghtt]

For starters im new to biking and have started trying to do it for the exercise so excuse my poor knowldege on bikes. I have had my huffy mountain bike for years stored in the garage never touched once until today. I filled the tires with air and then when i proceeded to try to ride the bike the chains would clank. The video shows all the problems im having visual and whatnot. It's especially prominent when im backpedaling.

View: https://www.youtube.com/watch?v=Ua2ilU_nu7A