You have to know every civ traits and some tricks at your expense : war cart rush, scythia horsemen rush, legion rush, but also caravels rush, or other naval UU rush (oh my god, I learned this one the worst possible way : it was an archipelago map, Brazil was next to me in another archipelago, he rushed his civics unique unit I wasn't even aware of, one shoted my entire fleet in one turn, then he probably conquered my cities one by one since they were all coastal, with campuses and library in every of them. I was far ahead of him in science, but firaxis decided to insert some weird social policy unique unit in the tree for Brazil, it looks so silly)
If you want to go multiplayer, you have to beware the hosts that want their little victory the way they planned it. Best is random civs and no barbs. (you still have to protect your settlers though)
You better be able to win Deity in SP though.
Oh, and if your enter a random game, you probably will be kicked without a word, for some reason I never elucidated. I only join games that mention "welcome all", and create only games with that mention too.