Ohm's law is the smallest true sentence in electrical engineering, and you will use it more than everything else in this course combined. $V = I R$, read aloud V equals I R.

Name the letters once and never again: $V$ is the voltage across the component, in volts; $I$ is the current through that same component, in amperes; $R$ is its resistance, in ohms. The prepositions are load-bearing. Voltage is always across, current is always through, and the two must belong to the SAME component or the relation says nothing. Whichever of the three the question hides is the one you solve for: $V = IR$ going one way, $I = \dfrac{V}{R}$ and $R = \dfrac{V}{I}$ coming back.

Two habits worth building. First, sanity-check by direction: raise the voltage and the current must rise; raise the resistance and it must fall. Any rearrangement that moves the wrong way is wrong, and you will spot it faster than you can re-derive it. Second, push the units through: $\mathrm{A} \times \Omega$ is exactly a volt, so $V = IR$ survives the check. If the units refuse to cancel into the answer's units the rearrangement is wrong, no appeal — but units that do work out never prove you right.

One nugget for the lab: resistance is a property of the COMPONENT, not of the circuit it happens to be sitting in. A 47 Ω resistor is 47 Ω on the shelf, on the bench and in the panel. Voltage and current are what the circuit does to it.