A lot of it depends on your civ and your pantheon, which can alter things. It also depends what you want the city to do, some locations are good for growth, others for production, but generally you're looking for:
1) Lux's. If it's your first city, ones you can acess via a tech tree path you would be taking anyway are the best. land generally quicker than sea. If it's later cities, new Lux's or Lux's you can trade are useful.
2) If not the 1st city, any strategics you want and don't have.
3) Bonus resources. If they are upgraded by a granary then all the better. Or if all the bonus resources are upgraded by the same building, e.g. all fish for a lighthouse. In your 1st city especially, number of rings from city location is important too.
4) River. Gold, strategic defence, and civil service. Also lets you farm hills if wanted. Can build watermill.
5) Settle on hill. +1 hammer from the get go, and higher city defence.
6) Settle adjacent to Mountain. Allows Observatory. A couple of mountains also provide locations for MP and Neusch. Close Mountains also provide strategic defence.
7) Natural Wonder if not 1st city (or if settler rush!).
8) Forest, especially in 1st city or if on Tundra. Help no end with early wonders llike NC.
9) Coast. Canbuild ships. Usually help with defence, but not vs. say Betty.
Things that can go either way/not really good or bad:
1) Jungle. Great science yields later on, but big barrier to growth early on, and building on it / chopping it is very slow, gives no added production like forest.
2) Grassland/flood plains. Good for growth, not production.
3) Hills (except on snow). Good for production, not growth.
4) Plains. Somewhere in between.
Bad things.
1) Marsh. Low tile yield, weak units, slow movement. However, can be used well strategically in war, esp on defence.
2) Tundra, no hill, forest or resource. Low yield.
3) Snow or flat desert. No yield.
There are, of course many exceptions and additions to these, like likely locations of NW's, strategic resource density, locations for good pantheons etc.