I did a copy past on a website onto Libre office and ever since then, every first letter of a word is capitalized. I don’t know how to fix it.
I guess that you have copied a character format or a paragraph style with Font Effects set to Case Capitalise Every Word.
Shown in image above is a paragraph style with Capitalise Every Word as an attribute. Click in the paragraph and then right-click the highlighted paragraph style in the Styles Pane of the Sidebar and select Modify (or maybe Edit Style depending on version). Make the change in the tab shown and OK.
It could instead be a similar setting in Format > Character. If you are unsure about styles, you could try selecting the affected text and clicking Format > Clear direct formatting or press Ctrl+M. If the capitalisation does not go away then it is a paragraph style setting.