The opamp usually comes in the form of an 8 pin integrated circuit, the most common one being the type 741.

It has two inputs and one output.
The input marked with a  - sign produces an amplified inverted output.
The input marked with a + sign produces an amplified but non inverted output.

The opamp requires positive and negative power supplies, together with a common ground.
Some circuits can be designed to work from a single supply.

If the two inputs are joined together, then the output voltage should be midway between the two supply rails, i.e. zero volts.
If it is not, then there are two connections for adding a potentiometer, to remove this OFFSET.