Right now I build cities approximately 8 squares apart most of the time. The exact number depends on terrain and resource locations. This will give you solid influence coverage relatively quickly and allow room for your cities to grow. You should be placing new cities near any resources you find if they are more than 5 squares away from your city so they have a chance to be linked to a city.
In general you should build just about everything in every city until you get a firmer grasp of things (pay attention to upkeep costs though), but even then most buildings go everywhere.
If you still have observers either A.) research weapon techs, or B.) design your own units. Actually design your own units anyway because the ones it gives you as a default are pretty badly designed all the way through.
As for research, it really depends whether you are playing kingdom or empire, how you want to play and how a game is progressing. It's also not exactly intuitive where certain techs can be found. Like for kingdoms the housing tech is found deep in the tech tree on a branch that is completely disconnected from the food techs. I would recommend just playing a game on an easier difficulty (they are all pretty easy, even ridiculous) and then dragging the game out and just trying things. For example, I only just learned there is a level 10 summoning spell "Pariden's Return" that requires researching up the magic tech line (kingdom) that summons a REALLY awesome titan unit for 5 turns. I never knew that before because I just never went that route or dragged a game on long enough to explore it. I've also never really gone very far up the weapons/armor line of kingdom techs because I get by with bows and summons. So basically just play around and see what does what for a few games and you'll start to find things you want to aim for and that will guide your early game.
Not running into opponents on turn 123 is hard to assess. 3 opponents is pretty low for a large map, but if you're on a tiny map you need to explore more. Also opponent starting location is random, some games you start with all of them clustered around you, other games they're clustered on the other side of the map, and everything in between.
