Hello and welcome! I hope that you will enjoy your time here, don't be afraid to ask any questions c:

Greetings. =]

To answer your body language question, there isn't really a system. You just know what you mean and try your hardest to convey that to others.

However, some things are seen as aggressive actions if you know what you're doing and you're not in-character. You can decide what actions are appropriate for you and/or your deer. :]

There are a few common 'movements' most players understand. However the 'common body words' usually have multiple meanings. To understand which meaning they are trying to express you must watch their behavior.

1: Bowing: Usually used as a greeting, a goodbye, and a thank you. Think of Hawaii which uses Aloha as both hello/goodbye. Usually for a goodbye the bow is used with the 'sad' emote.
2: Pawing the ground: Usually used to say "follow me", "sit here", or "I want this spell". Again other signs will tell you which it is. If they want a spell they are likely pawing at the item that gives you that spell.
3: Taunting often used with lowering antlers and rearing: Aggression. This is normally used in character for role players. Taunting is often seen as an angry emote. Of course some players will goof around with it. When they are just playing they normally add the "laugh emote" to show they are not serious. (Rearing by itself can also be considered a playful emote.)
4: Laugh Emote: Basically what it says. : )
5: Nuzzle Emote: Affection, of course. : )

All other emotes tend to have a very wide spread of meanings. Some players even use two emotes in a row or together to change the meaning of the first emote. Making it almost look like a new emote. The best way to know what it means to a deer is to get to know the deer, to watch other signs in it's behavior. If you plan on using the community a lot some players make a list of what emotes they use and what it means to them.
For me personally one of my favorite things to do when first joining the forest was learning the forest language and it's array of dialects. : )

Anyway, Welcome to the forest!

Veegamer also has this old blog that has very deer-like interpretations of some of the actions available in the game. I used this as reference when first starting the game. Not to worry, though - most of it you'll learn just by watching others.

Hope it helps. ^_^
http://endlessforest.org/community/translation-virgil039s-deer-language-gt-english