Huffy stalker 18 speed, I can't tighten the stem.

Loosen the bolt in the center of the stem. Tap it down with a hammer to release the wedge. Position it where you want it and retighten.

There is a bolt at the top of the stem (the part that fits in to the fork and clamps around the handle bars) which is the only thing you need to loosen to raise the handlebars.
It should only need to be loosened 2 or so full rotations. (If you have loosened it a lot, tighten it back up a bit.) The secret is that you need to bop that bolt with a mallet. (The stem is held in place by 2 wedges- they jam together to make the connection secure. Sometimes they get a little stuck.)
If that doesn't work, try loosening the bolt another turn and bopping it harder.
(Make sure that you don't raise the stem up any higher than the "insertion point"- the series of lines on the stem to mark it's maximum height. And make sure that you don't damage the bolt from hitting it too hard.)
If that doesn't work- I'd recommend taking it to a shop. They shouldn't charge to much for the adjustment... all the shop's I've worked at would do it either for free or for under $10.

Did you try a search engine?

http://www.huffy.com/Support/OwnersManuals.aspx

A simple web search for "how to... " just about anything will find an answer. "tighten bicycle stem", for example.

http://www.sheldonbrown.com/handsup.html

Stop guessing. You've used up three of your lives....

Search engine "how to adjust threadless stem" for videos.

Is that the stock stem or did someone replace it? It almost sounds like the inside diameter of the stem doesn't match the steerer tube, or something's cracked.

http://sheldonbrown.com/handsup.html

http://archive.mtbr.com/09/0EF97936.php

One person had this to say about it... "It was a two-bolt stem, and the problem was that I wasn't tightening the bolts to the same torque setting. If you don't do that, the stem doesn't clamp the bar evenly all the way around, and it will start to slip, and the uneven forces will cause the bolts to loosen.
"

Tighten the bolt that is in the center of the handlebar piece. As well as the nuts/bolts on the lower part of the shaft that comes up and connects to the handle bars. Hard to explain. But just align things right, and tighten everything up on the entire wheel/handlebar assembly.

That's up to you and your abilities and available tools...

http://www.parktool.com/blog/repair-help/categories/hub-service

If you just need to raise the height of the stem, all you need to do is loosen the hex nut at the top of the handlebar stem with a box wrench or adjustable wrench. This is the head of the stem bolt. Once loosened enough the stem should slide up and completely out (if necessary) of the stem casing. There is a tightening mechanism consisting of two soft metal pieces that tighten against each other when the stem bolt is tightened. When this is loose you can raise the stem to various heights. If you find you cannot raise it to your desired height you may need to replace the stem and bolt. However, if the bike has been sized properly to fit your leg length, you should have no problem there. Once the height of the stem is satisfactory just tighten the bolt while keeping the stem straight and you'll be ready to go!

single speed bikes have a single ratchet or fixed sprocket with no offset of the spokes, so you would probable need another rear wheel.
Another derailleur is probably simpler, try ebay

Sounds like the valve stem you may need a tube if you bought it new or recently new take it back??Hope this helps?