Look in the top right corner of any answer on your question. There will be a faded green checkmark - if you click on it, you'll select that answer as the best answer (BA).
There is no points requirement for selecting BA. You actually gain +2 points for doing it!
If you do get an answer that totally resolves your question, please select that answer as BA. If no answer is selected after some time and the mods think there's an answer that should be selected, then they can do it themselves.
You can only see the green checkmark if you are the original author of the question (or if you're a mod). Selecting the BA isn't a permanent action - you can de-select an answer or select a different one at any time.
Having your answer selected as BA awards you +20 points. You can read more about the points system here.
