what kind of sound are you looking for first of all
there is ported and sealed (don't **** with making your own bandpass box)

sealed is the easiest. just make a box and throw the sub in (but its more than just that)
for your sub i'd recommend anywhere from 1cuft to 1.75-2 cu ft, taking into consideration that the low end extension will be easier to acheive as the volume of the box gets bigger.
sealed generally provides a tighter bass response but less high end response. what you can do to figure out the volume of the box is multiply the width, height and depth of the box and divide that number by 1728 and it will give you a number with a decimal. for instance, 14*14*16= 1.82 (cu ft). probably pushing it for a sealed box application in terms of airspace. i'd personally go with 1.3 cu ft or so with your sub. so 12*12*15=2160/1728=1.25 cu ft. i'd recommend going with this volume for a sealed box for your sub.

the reason you divide by 1728 is because that is a cubic foot. 12*12*12=1728

now ported gets more complicated. also known as vented, ported enclosures have a port or vent to let air escape from the enclosure as well as be drawn into the chamber. ported enclosures are louder than sealed enclosures. i'd recommend probably about a 1.5-2 cu ft enclosure for your sub with a ported application. i'd tune the port at about 25-30 hz or so. you want to get the ported effect but still have sound quality. once you start tuning the port to higher numbers (ie 40-60 hz) you inversely loose sound quality when the tuning frequency gets higher. how the tuning of ports is determined is the area of the port. if you're just beginning i'd recommend you'd have someone build the ported enclosure for you given the tuning frequency i recommended and whatever volume you'd like within the dimensions i recommended.

take into consideration that the bigger your enclosure is, the less power it will take for the sub to reach full excursion. if you stay within the dimensions and tuning frequencies i've said you should be perfectly fine running the RMS wattage to the sub.

how sturdy and durable your enclosure is is probably the singlemost vital aspect of your system. you can have shitty gear and put it in an awesome enclosure thats sturdy as hell and built quality, and it will slam and sound good. but you can put top knotch gear in a shitty, poorly built enclosure and it will generally sound like shit. i can't emphasize this enough
no matter who builds your enclosure, make sure that it is built of 3/4 inch MDF (medium density fiberboard) or a stronger material. plywood=no
make sure also that the enclosure is braced well on the inside (the walls are braced together to reduce the flexing of the walls)
hope this has helped you and i'm sure others will respond. feel free to ask questions

edit: the jbl 300.1 would be a great amp as long as you have a dual 4 ohm voice coil sub. you can wire the coils in parallel to present the amp with a 2 ohm load. at 2 ohms the amp puts out 300 watts RMS. perfect for the sub. but if you buy this amp, make sure that you go with a sealed box as this amp does not have a subsonic filter (basically, if you go with ported, this amp will make your bass sound kind of muddy)