3. Is there any resources that are easy to fall in love with but isnt that usefull?
4. does a city have full cap when its working as many tiles as it possible can? and is it very bad to have 2 cities to cloose to each other? and what is the minium distant of tiles that should be between them?
3. Gold. Early on (first or second city), Gold can put a serious dent in the research times for the first few techs.
On a side note, desert tiles are useless until Oil appears. I will try to expand my borders into the desert and hope for the future oil (if your games last that long). If your economy can handle it, a lone city in the desert can cover alot of ground. The same can be said for the tundra tiles.
4. You need a population of 20 to work every single tile (and no specialists). Depending on your play style, your cities may never get that large. It takes a lot of buildings and connected happiness/health resources to get a city that large which doesn't tend to happen until the late game.
So if your city only has a population of 10, then half of the tiles are not being used. Re-arrange the citizens to allow overlapping tiles to be shared. Also, there are some cities (desert, tundra, hilly) that might only be there for resource purposes. If tiles overlap with a "good" city, then sacrifice the resource cities growth/production for the benefit of the good city.
Depending on how much you micro-manage, you can also share food resources between cities to grow the city to your current health/happiness cap. If your city cannot grow, why work that tile?
Lastly, you can reach a point in the game that you have 6-7 tiles that are empty between cities. It might be beneficial to place a city there and let that city work those few tiles.