No, Krypton was not made of Kryptonite - at least, not initially.

Kryptonite is a radioactive mineral from Krypton. The most common form is Green Kryptonite, but it comes in other forms, including Red Kryptonite, which has unusual temporary effects on Kryptonians, but only works once, and Gold Kryptonite, which would permanently remove Superman's powers.

Depending on the era, either Kryptonite was created by the explosion of Krypton, or it was found in the core of Krypton, and was released when the planet exploded.

The radiation from Green Kryptonite is poisonous to Kryptonians, causing weakness (removal of super-powers) and effects similar to radiation poisoning in humans. It's not quite the same, because Kryptonians recover very quickly when the radiation source is removed.

Reference information here: http://smallville.wikia.com/wiki/Kryptonite