Basic V meter with load
What you are looking for here is Ohms Law. This is how you would find your resistor value. 

The Law states that V = IR, where V = Voltage, I = Current and R = Resistance. 

You know 2 of your variable, the Voltage (8V) and current (1A). All you need to do is rearrange the formula to solve for R and you have the resistance value you need to simulate your load.

You will also need to think about the power dissipated in Watts (Thanks @Jasen for mentioning this). Once you have your resistor value, you can calculate Watts with the formula P = IV, or any variation of this (P = I^2*R, P = V^2/R). Once you have your answer (in Watts), be sure to make sure your resistor can handle it.

An example is if you have 10V across a 1 ohm resistor. Ohms law tells us that there will be 10A of current. These values give us 10W of power dissipated. This means your resistor is going to get hot! It also means you need to select an appropriate resistor, so if you get the first one you come across, it may only be a 500 - 700mW resistor, in which case it gets burnt out.

So to summarise, calculate your needed resistance value, and then calculate the power dissipated. Once you have these values, you will be able to select an appropriate resistor.