
Welcome to the Madness....and to CFC...or maybe that is one and the same.
Anyway, it sounds like you may be fairly new to the game. CIV is a strategy games and that is somewhat synonymous with having a plan. It takes some time and practice, but once you get the basics down you can have a better idea of what you need to be doing at a given point in time and set up strategies towards achieving your victory condition.
Yeah...that all sounds pretty generalized. However, your questions are very general and open ended. There's not much I can tell you with out having a better understanding of your game, level, etc.
I encourage to mosey on over to the Strategy & Tips forum as well as the War Academy to read some articles. The S&T forum is a great place to hang out in order to learn the game and see what other more experience players do to win.
So a few points tied to your questions - very generalized:
1) Build/Research next. First, I recommend getting an understanding of city specialization, if you do not have on already. Again, articles and S&T can help or more pointed questions as to what you don't understand. The point though is to have cities directed toward a purpose, such as:
Burea Cap
Great Person FArm - high food Natonal Epic city
Production Cities
Heroic Epic city, if needed
Commerce Cities - usually cottages
etc
Once you figure that out, it will give you a better idea of what to build in cities. You don't need all building in every city. Hammer cities usually get Granaries and Forges. Coastal with Lighthouses. Plus, and other military building suitable for your needs (No horses/ivory=no stables) Commerce cities get buildings that boost research or gold as needed (Libraries, Markets etc) When you covered most of the basic buildings for the city type - often around Medieval - you might set your cities to building Wealth so that you can tech faster and win Lib or otherwise set yourself up for a winning strategy (military advantage, Diplo, space, etc).
Workers - You obviously won't plenty to get your land improved as fast as possible and trade routes built. If your reach a point where you land is all improve, which may never happen if you keep taking cities or expanding, you can a) start stacking them which minimizes dealing with some many workers and gets improvements up faster b) start automating them - usually trade routes. Just be careful with automating as they can get sniped and make sure you really have your land improved how you want as workers can do stupid stuff.
Also, note that better improvements show up later like windmills, watermills, workshops and railroads, plus late resources, so you still want those workers around.
How many cities - Are you not using your advisors. Press F1. There's a lot of info in the advisor screens.
Also, try the BUG and BULL mod. The provide a lot more info in the UI and improve the advisor and city screens. Very popular here. They do not change gameplay.
Are there any reminders what to do next or something? This question is really too broad to answer. It's really a matter of getting more familiar with the game, learning tips and strategies to win certain victory types and so on. Again, go to S&T and start practicing the game and asking more pointed questions. Eventually all this will come into focus. Trust me, we have all been where you are at one point.
How do you manage all this stuff? see above