If you are getting an adult greek tortoise, a 40 gallon tank isn't big enough. When you measure aquariums, the "40 gallon" size includes how deep the tank is. You don't care for depth, what you want is floor space, and an adult tortoise requires a bit of room to walk around.
Yvonne is right, but if you're getting a baby, 40 gallon tank should be enough for a little bit, but eventually a 4ft x 4ft minimum is necessary when it is an adult. I'm building a 6ft x 4ft for when I get a baby. You will need a substrate of some sort, Coco coir, organic topsoil, orchib bark or cypress mulch are all good ideas. You will need a place for it to bask, and UVB, you can get two seperate bulbs or an MVB. Do NOT get a coil UV bulb. you will need a humid hide, and some rocks or other things for it to climb on. You will need something to measure the temps, a temp gun is good, and something to measure humidity. Somebody should say if I missed anything.