Reputable websites have the following characteristics:

• one of these domains - .gov, .org, or .edu (except webpages of individuals)
• author(s) and qualifications are available - look for About Us
• authored by an organization or institution rather than an individual
• information has a current date of publication - see bottom of pages
• proper attribution given for quotes, paraphrases, and images
• evidence or citations provided to support arguments
• lack inflammatory language
• professionally done web design, grammar, word usage, spelling, etc.

What about Wikipedia? In general, it's not a credible website because individuals write the entries and don't undergo rigorous review. But the entries can be a good starting place for understanding a topic. Use the external links or references in an entry to progress to more credible sources.