Encyclopaedia like Wikipedia and Britannica can be trusted nearly always. However, do not trust them blindly. Look for other sources to verify what they say, especially if the accuracy of the information is critical.
Besides these two, there are many websites on the internet. How do you decide whether to trust it?
Look if the page has 'comments'. If they exist, are they in support of the material or against it? If they don't exist, try knowing more about the author from sources other than the author himself. Or, you can try using a web search engine to search about the topic the page is about and check if more sources support what is written on the page. Check if any of those pages allows for comments and check the comments.
Try understanding this to convince yourself that the earth is not flat. If you don't understand it, you may return to it once you know some more of geometry and science.
Besides this, in the case of news, it can be useful to look at several different sources to frame your opinion. You would want to rethink this as you grow older.