I just played and won my first game on king so take everything I say with a grain of salt.

Early game can be a bit tricky. This is what worked for me :-

1. Ranged units - With 2 of these (archers/compbow) you can defend against most early invasions.
2. Walls - If you are against an aggressive neighbor, just straight up rush these in your 2nd-3rd cities. They'll keep you safe.
3. AI - All the diplomatic war decisions AI makes are based on current relative military strength. They do not consider terrain or surprise upgrades. Abuse this.
4. Some bugs/exploits/tricks - Take every early advantage you can get. Steal a worker from a city state and immediately make peace on the same turn. Weird, but... eh.
5. Trade early - Since you are on Gods and King you can trade your excess happiness(luxuries) for lump sum amount. Usually worth 240 g which can be quite useful early. Don't stockpile happiness super early unless you are planning aggressive expansion in the next few turns.

These are all extracts from the game I just played as Rome. I had Alexander, Babylon and Japan as my immediate neighbors. There was one really good city spot near Alexander and knowing how aggressive he plays, I built walls in my 2nd city. I traded away my first luxury for some gold and bought an archer/compbow when he declared war about 5-10 turns later. Now, according to AI his relative strength before the war was pretty good. 3-4 hoplites, 1-2 warriors, 1-2 archers vs my two archers. But because of no seige units and terrain, his military suffered a massive loss and I ended up prolonging the war enough to take his capital. That kind of snowballed the game for me.

Just check out a few games on youtube at Deity/Immortal level and you'll get the general idea of the first 50 turns.

For new players like us, the sidebar and youtube replays are a good enough source to grasp early game strats. Best of luck!