http://www.civfanatics.com/civ4/strategy/city_specialization.php has a nice section for
feeding your city for determining the amount of farms for "optimal plot usage" (one population working each and every plot) applies to production or commerce cities. Basically getting the most out of what you already have. Following it you can (happiness and health willing) have multiple 20 pop cities providing steady

/

/

.
Build all your needed farms from the get go.

=

with slavery. After you get biology convert extra farms to other improvements. When you found the city you should know the number of farms needed.
Things that can change from one game to another.
lots of food resources = whip for

regularly.
lots of food resources + happiness resources = whip for

like you click end turn

limited happiness resources = smaller pop to start so whip the unhappiness away

just not as often as with abundant food.
financial + lighthouse = sea squares worth 3

each (hello Vikings

)
rivers = +1

for all but forests (lumber mill recaptures this)
farmed plain = forested grassland = 2

1

If the plain touches river (not on corner) it will also get +1

.
farmed grassland = flood plain w/o improvement
The hardest thing to get used to... it is better to whip for hammers then turn off growth and just wait for happiness or health bonuses to fix it.
Best way to see this is run a settler game with a non financial leader (no

bonus) India is good choice for fast workers. At that low of level no one will war against you. You can mostly focus on the improvements. Try a couple times to see what difference bonuses and terrain types make. If you can't max out your pop on settler your doing something wrong. Also a good test for GP farms.