Hello and Welcome to AC! :hi: There is a number of things that can make for high nitrites. Have you recently changed the filter media? Really we need to know what you have stocked in your tank and the other water parameters as well.
Start a post in General Freshwater. List your tank size, what and how many fish you have, current ammonia/nitrite/nitrate/pH readings. And some info on your water change/maintenance routine.
Welcome to AC, I should point you to the respective forums for Fresh or SW help in the general Fresh or Salt areas, usually a large water change will help though