If it does not get natural sunlight for a couple of hours daily, buy UVA,UVB lights. I leave mine out in my garden to graze under the sun once in a while when I get the time like today for about an hour.
I don't use the UV lights, but then I don't have very many tortoises that have to be kept in either. Right now I have 11 little desert tortoises, 10 baby box turtles, a yearling Emys and a sick 3-toed box turtle that are all indoors. I just use a regular incandescent 75 watt bulb for their simulated sun. BUT I put everyone outside in the sun for at least three hours a week. Maybe not three hours all at once, but three total hours. Then as soon as the weather is good again in the Springtime, they all go back outside 24/7.
Please...I am NOT the be all, know all in the tortoise-keeping world. This is just what I do and what works for me. You have to keep an eye on your own tortoise and make sure he is getting what is required for his well-being.