but

first the feet, then the arms will follow.proper posture, hopefully how to hold your torso and pelvis properlyhow to work your feet through the floorhow to create proper turnouthow to move your arms (porte de bras) from position to position

typially barre work starts with simple tendus, ronde de champs, degages, frappes, rond de champs en l'air and some deep side, back and forward bends....then typically there is a break for stretching of the hamstrings and rotator muscles and so on...then developpes and grand battlements.

What is essential in all of this is that you keep breathing and relax...probably the most difficult part.  Make sure you talk to your instructor if you are finding any tightness in you body as you are probably doing something incorrectly.  Soreness the next day is typical depending on what part of the body the barre work is focused on.

depending on how long your class is...typically there is 1 hour of barre work and 45 minutes of floor work.  On the floor you may learn balances, the 9 directions and positions and variants as classes progress, piques and pique turns, possibly combination turns and so on.  there are hundreds of movements in ballet that are "named" then there are transitional movements that just sort of happen but are far from "break through".  

Classical ballet may be tiring and taxing.  It takes years of work to get your body to move the way clasical ballet requires.  If however you go with the idea that your turnout may never be the best, you may  not be able to do more than a double turn and work within those parameters you will have fun while still being corrected and pushed to the edge at times.

I would also suggest you take yoga, kripalu or vinyassa versions to start lengthening your muscles and range of motion.  Then try some pilates which encourages and focues more on strengthening those same areas which are critical to turn out and ballet postures.  Most pilates instructors can create a good routine for you knowing that you are doint ballet or in combination with any other sport to increase range of motion, flexibility and strength.