Minecraft wiki entry for Armor_Stand, associated text: /data merge entity @e[type=armor_stand,sort=nearest,limit=1] {ShowArms:1}, which will change the nearest armor stand into an armor stand with arms.
/summon minecraft:armor_stand ~ ~ ~ {ShowArms:1}, which will summon a new armor stand with arms shown.
/summon armor_stand ~ ~ ~ {Rotation:[0.0f]}, which will summon a new armor stand that will face your desired direction (you have to change the "0.0f" to other numbers or else it will face the same way as a default armor stand, such as "90.0f" or "180.0f").