Jml, one of the biggest roadblocks that new riders have is NOT putting their weight forward. Being in the back seat means you have to fling your board from side to side, by using a lot of upper body counter-rotation and arm-flailing. Putting your weight on your forward foot and using the edge to initiate the turn is almost effortless.
Snowolf is the ranking expert on describing this, but basically the simplified view is this: You start by pushing down on your front heel or toe (depending which way you want to go). You're essentially twisting the board slightly by applying front foot pressure. The board will start to turn on its own. As soon as that happens, you shift the back foot the same way as the front to get rid of the twist and just lean into the turn and enjoy the ride.
A really good rider doesn't even seem to be doing anything, just standing on the board and balancing.
There's a lot of variations for different situations, and different techniques depending on what you're doing, but this is the basic idea for just plain turning back and forth. You build from there.