To reach step n, you came from step n-1 (took 1 step) or step n-2 (took 2 steps).
So: ways(n) = ways(n-1) + ways(n-2). This is the Fibonacci recurrence.
Base cases: ways(1) = 1, ways(2) = 2.
Build up from base cases using a simple loop. You only need the previous two values, so constant space suffices.