This tool isn't particularly complicated, but let me take you on a quick tour.
First, paste the text you'd like to analyze here. I'll start you out with the UN's Universal Declaration of Human Rights.
Enter a list of stop words (words you don't care about) here. You can separate them with newlines or spaces. I'll start you out with a sensible list of defaults.
Click the big button.
The frequency analysis will show up below.
Whenever you change the text or the stop words, just hit the button again and the analysis will be re-run.