A 120 ohm resistor might make them a little dim, only giving them about 15mA.
I would use an 82 ohm, 1% resistor which will give them about their rated 20mA.
Connect one leg of an LED to a resistor, then the power goes to the other side of that resistor (thus, the resistor is in series with the LED) and to the other LED leg. Remember that the LED must be connected the right way around - the long leg goes to positive.
Regarding using the iphone plug - if it is rated for 1A then, as klystron says, you could power up to 50 LED's from that (1A/20mA = 50) BUT...
The charger plug might get hot if run at its full-rated current all the time. In normal use (charging an iphone) it would not be, so it depends how well it is built. If you go for the full amount, keep an eye on the plug and see if it seems to be getting uncomfortably hot. If it does, it probably won't live long. Then again, it might be just fine. Worst case, if it gets too hot, buy another 5V supply rated 2 or 3A for a couple of quid from your favourite auction site.