Tadasana: Mountain Pose
Stand upright, with feet parallel and hip distance apart. If this is comfortable, stand with big toes together, heels slightly apart. Choose which stance feels more stable for you.
Spread your toes pressing down through the mounds of your big toes and little toes, and press down through your heels; stand on the bones of your feet! From here, lift up through the arches of your feet all the way up to your knees. Continue lifting up your kneecaps as though you were zipping up the inseams of your legs, all the way to your inner thighs. This will help to lift your front thigh muscles and bring strength and stability to your legs.
Press the thighs back to the wall behind you so much that it may feel like your bum is sticking out. To counter the pressing back of your legs, draw your tail bone down to your heels to bring your pelvis to a neutral position.
From here, lift your ribcage up away from your waist to bring length to your torso and spine. Try to lift the front and back and sides of your ribcage evenly so you're not puffing your front ribs out too much, which can cause tension in your lower back. Lift up through your armpits slightly to bring a little extra buoyancy to your chest! This will allow you to breathe more fully into your lungs now that they are supported by the lift of your ribcage.
Widen across your collarbones and draw your shoulder blades gently down your back. This will also support the lift of your heart and lungs, allowing your breath to be easy and steady.
Lengthen your arms down along the sides of your body bringing a little energy to your fingertips so that your arms and fingers feel long and extended.
Finally, lengthen the sides of your neck and the crown of your head to the sky to extend your spine even longer.
Hopefully you will feel your whole body stable and energized, with your breath moving freely. This length through all your limbs and torso help bring length and extension to the spine. Tadasana, mountain pose, is the template for all of the standing poses you will practice in
classes and in your home practice.
A great resource for postures is
Yoga Journal. Their website has clear pictures and descriptions of asanas (postures) and how to practice them alone and in the context of a practice sequence. You can even build your own home practice on their website!