Here's how to display arms on your armor stand:

1// Type in the following command in creative mode to summon an armor stand with arms --> /summon armor_stand ~ ~ ~ {ShowArms:1} --> can hold items

2// If you're in survival, you can simply right click on an already placed armor stand with a name tag renamed to 'arms' or 'armed' which will give you a similar result. 
However since this only changes the texture and not the actual model's properties, you won't be able to give it an item to hold like you can with a summoned armor stand.