Rear derailer losing chain tension on Schwinn

[Toyeboy]

Hello, i just need help with this brand new Schwinn 21spd bike i bought from a department store. Please don't tell me just to get a better bike b/c a lot of people have told me that already, i just want to get this bike working b/c it's nice enough for me aside from this problem and I think it just needs adjustments.
okay so here's my problem on the rear derailer...when I pedal the bike it pedals good and easy, but once I stop pedaling just to coast, the (bottom) tension pulley on the rear derailer swings up and causes the chain to loose all of it's tension and the chain flips around and comes off. This happens for no obvious reason but the tension pulley isn't keeping tension on the chain when I coast and it's unrideable in this condition. Also I can't get the twist shifter to twist into 1st gear for the rear derailer so something is out of wack back there. please help!!

[Toyeboy]

hey i got this issue resolved, I took it back to the department store I purchased it from and exchanged it for the exact bike. The new one does not have this problem and it turns out the free wheel hub was grinding, I knew it sounded loud but this new bike is very quiet and nice. They must've not greased the free wheel hub or let it sit out in the rain or something.