I have a little dark spot on the left side of my tongue. what could it be?

BIOPSY: A red or white spot could be pre-cancerous or cancerous and a black spot could be black hairy tongue -geographic tongue. A biopsy would alleviate your fears.

Many things: A dark spot may or may not be something serious. The only way to be sure is to see a dentist for an evaluation. Consider if there is any pain or tenderness associated with the lesion? Is is flat or raised? And finally have you realized it has persisted beyond 2 weeks? I wouldn't get too excited from answers here as we are limited with just text questions, so see a dentist just in case.