Could somebody explain servicing multi-gear bicycles again

I rode mine to work (have you seen the price of gas lately) and we had a bicycle maintenance class but I think I screwed something up because when I downshifted the chain fell off and jammed itself between the smallest sprocket and the inside hub but there's a bolt there that holds the rear forks on and if I try and lift the chain up past there I'm going to bend it and possibly snap it which means I have to take a taxi home so I'm going down later with a 15mm socket if I can scrouge one from somewhere and loosen off the bolt and get the chain loose and tighten it up again but I need to adjust the rear derailleur so that doesn't happen again and there's two screws there but I've got a #2 Phillips already and I think it's the back screw you need to tighten up to move the derailer towards the inside but when I do that there's so much play on the cable that it won't go into the low big gear properly but if I tighten up the cable it's almost impossible to get it into high little gear and it's not the cable because the guy looked at it and he says it's fine so I'm wondering if it's the other screw that needs adjusting and I can back off the cable tension a bit which would take care of the back gears but the front ones aren't right either because it's taking waaay too much twist to get into high big gear and it drops into low little gear when the knob is only turned about half ways back but if I take up the slack in the cable it gets really hard to turn and I just about put myself over the handlebars doing it and no I can't afford a new bike.

little screws you'll see which limit the range of the derailleur.

get the chain unwedged. you might have to take the wheel off to do that.

if you take the wheel off, leave it off for a bit and keep the bike upside down while you play with the gearshift lever so you can find the limiting screws I'm talking about.

If your breakdown is not caused by a cable snap or a bent derailleur, then you need to screw IN the limiting screw a bit. don't screw in the limiting screw until after you've positioned the derailleur over one of the middle sprockets. then test the range of the derailleur after each adjustment. test by turning the pedals and shifting gears.

if the lever no longer moves derailleur once the chain is free, then you might need a new gearshift cable. that's tough to replace w/o some time, experience, and a pair of pliers, etc.

anyway take it easy on the gearshift. always be pedaling when you shift. don't shift too hard too fast or you'll snap the cable.

hopefully everything is well oiled, etc so the derailleur moves w/o much force.

be patient and tinker with it. good luck!

might be a good time to take it down the shop. Of course, if you don't want to leave your ride in the hands of some stoner for three days, maybe the Park Tools site can help you along.

http://www.parktool.com/repair_help/FAQrindx.shtml