The current, potential difference and resistance of a component are linked by one equation:
potential difference = current × resistance, or V = I R
Potential difference is in volts (V), current in amperes (A) and resistance in ohms. The equation can be rearranged to I = V ÷ R and R = V ÷ I. For example, a 12 V supply across a 4 ohm resistor gives a current of 12 ÷ 4 = 3 A. A component with 6 V across it and 0.5 A through it has a resistance of 6 ÷ 0.5 = 12 ohms.
For a fixed resistor at constant temperature the value of R stays constant. In some other components R changes. In a filament lamp the resistance increases as the current increases, so you cannot use one value of R for the lamp at every current. You calculate R for each pair of readings of V and I.