The international standard state that all OBD II connectors should be within 3ft of the driver's seat and accessible without employing the use of any tools.

Start by looking under the driver side instrument panel.
Do not forget to look under flaps, some card like Toyota and Hyundai have a small door cover.

Every brand and model of car can be different, but they are almost always located on the driver side.

If in doubt ask a mechanic or look in your owners manual.

You can also do a simple search on google with the make, model and year of your car like in the example below.

You can also use this site Where is my OBD port 

This is what the OBD port looks like