Validation error how do i fix this

I have been trying all day long to figure out what this means and i dont know , can u help me please someone. thank you- Tentatively passed validation, 1 warning(s)

Potential Issues
The following missing or conflicting information caused the validator to perform guesswork prior to validation. If the guess or fallback is incorrect, it may make validation results entirely incoherent. It is highly recommended to check these potential issues, and, if necessary, fix them and re-validate the document.

No Character Encoding Found! Falling back to UTF-8.

None of the standards sources gave any information on the character encoding labeling for this document. Without encoding information it is impossible to reliably validate the document. As a fallback solution, the “UTF-8” encoding was used to read the content and attempt to perform the validation, but this is likely to fail for all non-trivial documents.

Read the FAQ entry on character encoding for more details and pointers on how to fix this problem with your document.