-What guns require a permit? I read that a small caliber hunting rifle doesn't need a permit. Are there other exceptions.
This depends on what you mean by "permit." In the United States, firearms laws are divided into federal laws, which apply to all the states, and state laws, which apply only in the states where they are enacted.
All firearms buyers in the United States must fill out federal forms and pass an "instant background check" when they purchase a firearm of any kind from a federally licensed dealer.
In the United States, handguns are regulated more tightly than are long guns. Many states require a license for possession of a handgun, though it is much easier to own a rifle or a shotgun.
Because of the differences from state to state (and even from city to city), there is no one answer to your question. I have a pistol permit for New York State that is invalid in New York City, for example, and which does not apply to other states. That means I can carry a pistol, but not into NYC, and not across the border of my own state. One exception would be Vermont, a state in which no permit is required for handguns.
-What guns are available to ordinary citizens? I read that any citizen can acquire assault weapons like AK-47 and Uzi.
This is a product of media misinformation. An
assault weapon is, by definition, a
select fire weapon capable of automatic fire.
No private citizen in the United States may easily possess such an automatic weapon. While it is theoretically possible to become a "Class Three" license holder and possess such arms, it is not easy and it is not common.
Semi-automatic versions of various "assault weapons" -- that is, rifles that are cosmetically similar to such military arms but which fire only one round for each time the trigger is pulled -- are still legal for most citizens of the United States. The weapon in the picture below is a MAK90, a Chinese semi-automatic rifle based on the AK47. It is
not an "assault weapon."
Was the original reason behind the 'right to bear arms' to allow ordinary citizens to bear arms to overthrow the government in case the government would impose a dictatorial government or is this nonsense?
According to the writings of the Founding Fathers, this was indeed the purpose of the Second Amendment. The Framers of the US Constitution believed very strongly in the right of citizens to be armed, both for their individual defense and for their defense against tyrannical government.