How do I calculate output resistance of a simple resistive circuit?

maybe..but I don't really understand your question...do you have a link to a similiar problem?

you have to find the Thevenin equivelant resistance,,,

this is done by turning off all the independent sources( V source becomes short circuit and Current sources become open circuits) then you find the equivelant resistance beteween the two terminals of the load.
If you have dependent sources in your circuit you have to connect a current source ( of any value) between the terminals of the load and then find the voltage across the terminals and divide it by the value of the current source to get your Thevenin resistance

Input resistance - correct, provided you say "resistive" load.

The output resistance describes how the output vvoltage of a supply or device drops as you load the supply or device with current.

For example if you have a 10v supply source when the current is zero (open circuit) and you measure that after applying a load of 1A this voltage has dropped to 9v then at 1v/1A the output resistance (or with ac the impedance) is 1 ohm.

Generally, simply impose a small current or current change from the terminals of that circuit and calculate the amount which the terminal voltage changes. Delta v / delta i will give you the output resistance (d.c. circuit) or impedance (a.c. circuit).  

Ultimately, and even easier in the case of your resistance circuit which has only linear circuit elements, proceed as follows:
1. determine the terminals of your circuit at which you want to have the output resistance.
2. calculate the voltage across these terminals with no external resistance connected (open-circuit voltage). Call this V
3. calculate the current through these terminals when they are short circuited (short circuit current). Call this I.
4 Output resistance is = V/I.

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