a few really depending on lvl. the only issue with having lots of military is that it's pretty expensive in terms of upkeep, they need to be advanced too so it all needs to be balanced out. if the ai perceives you as weak then they will attack.
diplomacy is important in higher levels, as are the trades you choose to make, decide what win you want to go for early, if you go to war ensure you can win reasonably quickly - prolonged wars can cripple your economy and put you behind. don't neglect your science, use your specialists properly, choose the right wonders and er have fun!
civfanatics is a great forum for civ guides, it's as complex or as simple as you want to make it.