Bicycle Mechanics - Simple question from a beginner

I just bought my new mtb on Jan.1st, and after a little ride the chain began to go really slack when I would stop peddaling. While coasting there were a few times I thought the chain was gonna get in my rear spokes.I made some adjustments on the front and rear derailers, and although it helped with the shifting, it hasn't done a thing for the chain slack. Please Help!

Is it just at the bottom (chain going from crank to rear dérailleur) or both top and bottom? It might have something to do with the spring tension in the rear dérailleur.

-Middi-Zon

Does this happen on all gear combinations?

Seems like it wasn't slacked as bad in the lower gears as in the upper gears(21 speed)

If it is the spring tension in the rear derailleur, how can it be adjusted or should I take it to a bike shop and have it done?

What make and grade of components do you have on the drive train of the bike?

I don't know a lot about bikes and bike parts but I know that it has Shimano derailleurs, freewheel, etc.The bike is Shimano equipped.
So, if I'm understanding this correctly, this chain slack is not normal, right.

Well I took my bike to a bike shop to have everything tuned up.The rear derailleur is in bad need of adjustment.I will pick it up from the shop on Tuesday and then I'll get back with yall.
Oh Yeah, can someone tell me the best kind of front chain/sprocket guard to get? The one that is on there now is plastic and just broke right in half when I hit it with my foot. thx

You did the right thing taking it to the LBS for service, let us know if it still has problems when they are through.

You might be better off with a clip or strap on you pant leg than a chain guard on the sprockets. See what the shop has when you get the bike back.