Here's my top 3:

• Never leave your troops standing unprotected in your city. Always keep them hidden or in the infirmary. Free defense won't save you if a heavy offense player decides to hit you.
• Don't rush to unlock every agreement at once. Only upgrade the ones for units you actually plan on using.
• Join an active coalition on day one. Playing solo in Sparta is the fastest way to quit the game after a week. 

100% spot on about joining a coalition. Another huge mistake fresh players make is completely ignoring scouts. Never send your troops to attack or claim locations blind without scouting first. Also, stop letting resources sit capped out in your warehouses with no protection. If your stores are overflowing, you're pretty much putting a giant target on your back for every big player in the area to farm you. 

Hello Archon. Very good post. If I had to start the game from 0, the mistakes I would avoid are: building traps, having out defences, and leaving resources unspent, so I would avoid being attacked by predators, and avoid upgrading buildings that give little.  If someone attacks me, I would contact his hegemon asking him to be my ingame  mentor. It is very good diplomatic move, and maybe you will get invitation to join them too. My  first building would be the max level Acropolis and infirmary, and next the Bronze and the timber buildings, which can give me a % boost on my def and off etc . As for agreements I would go directly what is needed to open my Elysian city, and of course, upgrading my guardians.

I agree with you, Archon. Venators are the best for offensive scouting and kill the other player's scouts