Hold one of the three variable in the Ohm's law equation constant. Then change one of the other two variables and the third must change according the the equation.
Typical example is putting a variable resistor (potentiometer) across a constant voltage
power supply. In this case the voltage (correctly called E or U in Ohm's Law, E = I*R) is constant and the current, I (measured in Amperes) is dependent on the resistor value. Turn the pot, changing the resistance, and the current changes.
This is calculated by rearranging the equation to I = E/R.
Now to your example: If you put a resistor across a constant current
power supply. Then if you change the resistance, turn the pot, then the voltage will change as in E = I * R.
the resistor blocks current and voltage
resistors 'resist' the flow of current.
When a resistor is put across a constant voltage source, the resistors value does not matter (unless the source can not supply the required current). As in my first example the voltage does not change when the resistance value changes. Only the current through the resistor changes.
Two points to remember:
1- Voltage is the Potential across a resistor.
2- Current is the Flow of electrons through a resistor.
Your next task is to connect different value resistors to a voltage source, measure the voltage and resistance then calculate the current. Also calculate the Power (P = E * I) and feel the heat dissipated by the resistor (proof that the equations do work).