Use the wave equation: wavelength = wave speed ÷ frequency
Write it as: λ = v / f
If the wave is light in vacuum, use: λ = c / f
If the wave is on a string or in a medium, use the wave speed for that medium
If you know the period, use: λ = wave speed × period
If you know the frequency and speed, divide speed by frequency
If you know the distance covered by one complete wave cycle, that distance is the wavelength
If you know the number of waves in a given distance, use: wavelength = total distance ÷ number of waves