I usually recommend reverse camber boards for absolute beginners looking to buy, and if you were renting, I would have put you on a board shorter than 150 (145 maybe) but shortly after making progress the 150 would be better.
Based on your description of twirling around, it seems you already have identified your general problem. You need to figure out how your body works and learn how your bodily motions translate to the snowboard and your motion down the hill.
When I am teaching, and I see someone twirling down the hill, it actually shows they have a good center of balance, but they just need to apply it correctly! To be honest, that is usually the point that I grab their hands for balance, have them lift their head and look where they want to go (staring at feet/board leads to falling!), and allow them to transfer weight and pressure to different feet/areas of the feet to see how the board reacts. Soon they begin to get the muscle memory required to slide down the hill heel side and toe side and from there, they can take on turning!
I totally get that once the board starts twirling, its really hard to get it stable again, but you gotta get those eyes off the board, focus on where you want to be (if its straight down the hill then focus 20 or 30 feet in front of you), and keep calm.
The main thing to remember, based on what you describe, is that if the left tip of your board starts pointing downhill, the closet foot (left) has to do something to control it. If the right tip starts a downhill trip, the right foot needs to do something to control that tip and keep it going across the hill. Ex: Sliding down on heels, if left tip starts pointing downhill, then you need to pull up on left toes (pressure on left heel) to get that end of the board pointing back across the hill. Same goes for right tip and right foot.
As long as your weight is centered, 50/50 each foot, then this will work. If you are leaning on one foot, taking weight off the other, then the pressure applied to the unweighted foot will not impact the boards movements since there is no weight to back up the pressure on that toe/heel. Ex. Sliding down on heels, left tip of board starts to point downhill, if you (out of instinct to save yourself) lean away from the downhill foot (left) and start leaning uphill (weight on right foot since left is pointing downhill), then you can't control the board with left foot due to no weight backing up your foot pressure!
All in all, just keep at it! Go out there with one goal, read about how to achieve it, and take baby steps instead of just "generally" trying to turn, or trying to carve. There is a lot going on to get there!
Last edited by BigmountainVMD; 01-14-2013 at 10:00 PM.