Time would slow down the closer you get towards light speed. However. Albert Einstein said that even if you had a ship capable of that. That the width of your ship would expand endlessly. And the lenght of your ship would decrease endlessly. Basically meaning it isnt possible to go the speed of light because neither you or your ship would survive that.

They have done tests with atomic clocks. Which were perfectly in sync with one another. One of them they left on the ground. The other they put on one of the fastest jets they had. And noticed that time does indeed slow down. The difference was so small that it barely made a difference. But it proved Albert Einsteins theory.