In some ways, writing fiction is like acting. It requires a lot of getting into the head of a character and seeing the world through their eyes. Sometimes your characters are similar enough to you that their voice is easy to capture. However, stories tend to take their characters to the extreme, meaning they may, more often than not, have the complete opposite personality of their creator.
Think about a character you’re familiar with. This may be one you’ve created or a favorite character from a story you enjoy. Then, choose a method of transcription that seems true to that character and their world (i.e.: a diary/journal entry; a postcard/letter; an old typewriter/computer; a receipt; a scroll, etc.), and write a page worth of that character’s thoughts or feelings. These could be addressed to another character, or merely a stream of consciousness. Really get into their head and let the words flow naturally.