the link that i sent you, explains how to calculate the period and the frequency. It even has a quiz on it at the end and shows you how to calculate it properly. The wavelength in this case, you can consider it to be the same as the period. So don't get confused by terms. The scope will give you the period not the frequency, measured by counting the horizontal squares from point A to point B, being that the period that it takes for the wave to repeat its shape. A sine wave it will be two zero crossings. Then you take the number of squares, multiply it for the scope time scale (the know in ms or us) and then take the inverse of that. f=1/T that will be your frequency.
A frequency counter is ok, but you need to learn some of these basic concepts, buying equipment more and more will take money and in the end you need to understand these things anyway, there is a bunch of online resources that can help you on how to use these tools. I just looked it up and there are also youtube videos explaining how to use a scope, how to measure period, etc.