Welcome to Civfanatics btw
Usually when members request for help, they would post the difficulty, the map type(optional), the Civ they are playing as and most importantly, the version of Civ V they are playing. There is actually a huge difference between Vanilla, Gods & Kings and Brave New World and the tips given might not be universally applicable for all version.
Sorry to say that my Vanilla memories are extremely hazy.
You can ignore my advice about Mercantile City States and religion as they do not exist in Vanilla. Befriending City States is still a good idea though for the lux they grant. Be careful about conquering City-States, the other AIs will hate you and other CS will be become apprehensive. Conquering city-states also give you unhappiness unless it is offset by lux and courthouses. So do keep that in mind.
Do check if Protectionism is in commerce. There were a considerable number of policy swaps in the expansions so I might be wrong.
Unhappiness (at least how i remember it) have different tiers. If you have minimal unhappiness, it will stifle only growth but have too much unhappiness ( i think that is around -10?), your production and science output suffers. At -20, rebels spawn in your territories to make your life more miserable. Just as excess happiness adds to your Golden Age counter, unhappiness deducts from it.