Go to the resource section and look at the resource describing how to use LEDs. That will answer this question in great detail as well as pointing you to web sites that can do some of the calculations for you.
The short answer is that you should probably connect pairs of the LEDs in series and then connect a resistor in series with each of these. All of the resistor/LED/LED strings should be connected in parallel (like the rungs of a ladder).
Assuming you want the 9V battery to last a while, you might want the LEDs to run at between 1mA and 5mA (you should experiment first to make sure this is bright enough) For 1mA the resistors should be about 2.2 kΩ. for 5mA you would use about 560Ω and for 10mA 220Ω.
Remember that the total current will be 15 times this value, so if you use 560Ω resistors, the total current will be 15*5 mA = 75mA. A normal 9V battery will last about 6 hours, but the LEDs will get dimmer and dimmer with time. Even after 6 hours there may be a bit of a glow.