caveat - This applies to regular snowboarding - not alpine boarding (i.e. hard boots and extreme forward stances)
The most common habit of skiers starting to ride is to turn their shoulders forward to face their direction of travel. You want to ride with your chin tucked into your shoulder and your shoulders more aligned with the board than perpendicular to the board.
Here are some dry land exercises you can practice ahead of time. Many of these are boot drills that I commonly ask my first time students to do. We start out doing these in just our boots, then do them again with only the front foot in, then again with both feet in.
Put Your Worst Foot Forward
Stance selection, forward foot - Normally you want your power foot in back. If you are right handed, normally your right foot goes in back, but some people are right handed and left footed or vice versa. If you've played soccer or football, you'll know which foot you prefer to kick with. Skateboarders or surfers with experience riding with their power foot forward can rider this way on a board. Although it is slightly easier to ride "forward", snowboarders often learn to ride "backwards" nearly as proficiently. Left foot forward is called a normal stance. Right foot forwards is called goofy.
Stance selection, width - Stand with your feet about shoulder width apart, hands raised up even with your elbows. Jump up. Check where your feet land relative to how wide a stance your snowboard is set up. Anywhere close is ok, but my personal recommendation is to have the insides of my heels underneath the middle of the tops of my thighs. Slightly narrower or slightly wider is a matter of personal preference and body build.
The Toes Pose
Stance selection, angles - Some rentals with come with a "traditional" default stance where the front foot is slightly angled forward from 6-12 degrees and the back foot is angled either perpendicularly to the board (0 degrees) or less slightly forward than the front foot (3-6 degrees). The Burton Learn to Ride program recommends a duck foot stance of +6 degrees for the front foot and -6 (i.e. pointing towards the tail) for the back foot). For beginners, the stance does not matter a whole lot, but pigeon toed (front foot angled back, back foot angled forward) is definitely an uncomfortable and inefficient way to ride. For a dry land exercise, try positioning your feet in different angles and bending your knees up and down to see if you can find a preferred position. Don't worry if you don't.
The "Ahhhh" drill
Basic Posture - Stand with your feet about shoulder width apart, hands raised up even with your elbows and knees slightly bent, chin tucked into your shoulder. This is the basic riding stance. Look down at your knees and see if they are directly over the toes. Wiggle your front knee back and forth to see how far in front or behind the toes you can get them. Hold the knee as far in front of the toes as you can. Check your back leg to make sure that it is straight (no knee bend). This is the "ahhhh" position. It is useful for getting off the lift. Reverse the position (back leg bent, front leg straight - knee behind the toes). This is the "ayiiii" (higher pitched "ah") position. It is what happens to beginner riders who go too fast. When you are going straight down the hill, you'll find that you can not fix the "ayiii" position. But when the board is positioned across the hill, you can change from "ayiii" to "ahhhh" by sliding your hips forward. Practice this move. When you ride you can have your hands down by your sides if you have great balance or hold them out for added balance. You can have your hands held so they cover the nose and tail and the board or you can hold them so that they are just outside each foot's little toe. This is a style choice.
Doing the Twist
Turning with the feet - Stand on a slick surface with your socks on. Hold your hands behind your back. Turn your feet without turning your shoulders. Have an assistant your shoulders still if you can. Try turning by just moving your toes or just turning your heels. Next try turning your feet using both your toes and and your heels (i.e. the pivot point is under the middle of the foot). You get the most power by turning with your whole foot versus just your toes or heels. When the board is flat, turning with your feet will quickly spin the board in a new direction, but will not change your direction of travel. This can be helpful to get a turn started but dangerous if you don't quickly get onto your uphill edge.
Clap your Toes and Heels
Turning by edging - The main way to get a snowboard to turn is to let it turn all by itself. The curved shape of the edge of the board causes the board to travel in an arc when the board has the edge engaged in the snow. To get an edge engaged we need to move our center of mass from the middle of the board to over the edge while we lift the other edge off the snow. The trick is to do this with a body position that provides balance and allows for a way to correct if move too far over the edge. On a toe side turn, the key is to arch your back. The easiest way to feel this is to stand flat footed with your arms raised high above your head. With your arms still held high above your head move your hands behind your head, then turn your palms to face the sky. At this point you should feel your back arched and your chest sticking out. Now bend you knees. You should feel your hips move forward and your heels come off the ground quickly. If you goo too far, you can move your hips back to "stand up" straight and flatten your feet. Now try to get the same movement with your hands down by just sticking your belly out. Heel side turns are done by making a move similar to sitting in a chair. The butt moves out while the shoulders come forward and the toes come up in the air. Practice these moves by alternately "clapping" your toes or heels on the floor.
The Moon Walk
One way to help get turns started is to make the edge change move (toe to heel or heel to toe) with your front foot first. You can practice this by almost "moon walking". Stand with one foot on your toes and the other foot on your heels, then switch. Once you get the hang of this, try starting with both feet on your toes, then moving just the front foot to the heel first, then follow quickly with the back foot. Switch and start with your feet on heels to start, moving your front foot to toe edge first. When you feel comfortable with this drill, you will be able to start your snowboard turns with emphasis on the front foot first. You don't have to ride this way, but some people find this technique helpful. It works especially well on the Burton LTR equipment.
Whack a Mole
Prepare to jump, but stop, then return to a standing position. You need to use this kind of motion when you are riding. Make the move smooth and slow. When we are traveling across the hill we use a rising motion to help us start a turn until the board is facing directly down the hill. When we finish a turn, we use a sinking motion until the board is facing across the hill again (or even going back up the hill!). At that point we are in the low position again and ready to start the next turn. Whack the mole before you start your turn.
Oompa Lumpa Land
Safe falling - Get on your knees on a nice soft carpet with four feet of open space in front of you. Fall forward and catch yourself with your hands. This is how the most common injury (sprained wrists) in snowboarding happens. It is a natural and automatic way to fall. Don't do this while riding! Try it again, but this time, make a fist with each hand, cross your arms and fall on your forearms. When you land make a loud "oomph" sound on impact. This is important. If you catch a downhill edge while riding, you will not have enough time to think about what to do. If you think "oomph", you can fall on your forearms instead of putting your hands out. If you practice oomph enough, you can make this your automatic way to fall. It won't be pain free, but it will prevent a wrist injury.