Some tips for the early part of the game.
1) Despotism is a fine government during the early expansion phase. It is much underappreciated by many.
2) Any rapidly growing city (a city with food bonuses) can rapidly produce a lot of units by hurrying production.
3) A large army of horsemen can quickly blitz two or three unprepared enemy cities. Usually the enemy will agree to a humiliating peace within a few turns.
4) Foreigners are suspicious and smell funny. You want to kill them off by hurrying off improvements, e.g. a temple and further offensive units. Same goes for foreign workers. Make the lazy bastards join a city and immediately convert them to something useful.
5) Once you have broken the spine of another civ, make peace and finish them off later. Get as much tech and money as possible.
6) When a civ is very weak compared to you, try to make peace, trading newly conquered cities for other cities around the map. Then immediately retake the city that you just let go (have forces ready). If the civ is weak enough, they may be mad but they will make peace again.
7) If you want to launch a surprise attack against another continent that is shared between other civs, agree to a right of passage with one of them, stockpile units there, and invade across the border.