How do I find the frequency of a vibrating string?

This is a lot simpler than it sounds. First off, you should know that the frequency is the inverse of the period. In other words, to find the frequency of something you take 1/period. The period is simply how long it takes the system to complete one cycle ( so the frequency would be how many cycles are completed in a given amount of time). For your string, one cycle would be when the string is straight, then up, straight again, down, and back to being straight again. So to find the period, time how long it takes the string to do that. Then, to find the frequency, take one divided by the period.