[BlowFish]

ATR is simply the range (H-L) adjusted for the last bar close (If the close is outside the range of the current bar (market gaps) you adjust the range to last bars close.

14 ATR will be a moving average of the last 14 ATR's. This can result in rather tight stops so people use a multiple of this, 1.5 to 2.5 is common depending on how tight a stop you want.

Lets say the ATR is 10 pips for a 5 min bar. (ATR is in the same units as price because it is a range) Lets also say you determine you want to use 2*ATR to keep you in trades through some small wiggles. Obviously you will need to risk 20 pips (2 * 10) (actually it may be a little more as I believe you would add the stop to the bottom of the current bar). Once you know the absolute number of pips you need to risk (20) you can calculate the position size based on how much of your account you want to put at risk.

Google should reveal nearly all you could possibly want to know though Katsz is a good reference to get an idea of what changing the multiplier might do to your results.

Cheers.