Hey everyone, this is my first post and I am just starting to get into this game. My early cities begin to decline when they reach lvl 8 and all the citizens compain that it's way too overcrowded, I have 4 other cities and many farms and cottages. What can I do to counter this problem?

You need buildings and resources that generate health and happiness. When cities hit a certain size depending on difficulty level, they'll automatically become unhealthy and unhappy. Look around this forum, there's another thread where people give detailed advice on how to deal with larger cities.

Also, make sure you hit the limit growth button in the city screen before your happiness passes the maximum. Allowing a city to grow past happiness is hardly ever a good idea, unless you need the extra citizen to whip so you can build something with slavery.

With the limit growth button activated, the city will not grow even if there is enough food for it, so be sure to be careful to deactivate it when you have enough happiness to grow again.

Slavery is the all purpose solution to overpopulation! Put 'em in the pits, where they can be unhappy and not show up as red faces on your city screen.

And if you have a HUGE problem, whip some happiness buildings :)

lol thanks for the advice guys, now i just need to not piss off every other civilization i see, :-D