Ok, I loaded it up, played a year, who the hell am I to designate my enemy, and how do I follow my mission 'expand the empire's borders'? I really have no idea which direction I should move or really anything
You're Austria, your friends and allies ought be obvious. The whole of the Holy Roman Empire is your friend, your enemies are France, and the Ottoman Empire. Align yourself with Hungary, form dynastic ties if you don't already have them, and try to form a personal union with Hungary. The Ottoman Empire is destined to rule the east, and your job is to contain them over the long term. You need to think in the long term, not short term. Short term gains can easily be long term liabilities, and shot term liabilities can be long term gains.
Your job is to become and remain the Holy Roman Emperor. Keep relations in the German states high. As Emperor, your job is to maintain the status quo in the empire, do not let member states lose territory or be annexed.
Your mission, expand the borders of the Empire, means to use diplomacy to ask non member states to join the empire, or annex non-member territory and use a provincial decision to add it to the Empire.
As you gain authority as Emperor, you must start proposing reforms to Imperial government to centralize administration, and eventually unify the realm under your enlightened rule.
This will most likely not happen fast. Defend the Empire. Develop your country, forge dynastic ties across the empire, especially with Hungary and Bohemia.