I have a 2 year old horsefield and when indoors she is kept in a tortoise table. Mine is 4.5 foot x 2.5 foot- but the size you need depends on the size of the tortoise you are wanting to get. For a substrate I use top soil from b&q as horsfields love to dig around in it and I have found it to be the best option.
You need to provide heat and UVB. This can either be achieved by using a combined bulb- like this one I use
arcadia 160. This provides both heat and UV. Or you can use a UVB tube and a normal basking lamp. You want a good temperature gradient going from around 20 degrees celsius to 34-36 in the hottest area (or the basking spot). To monitor this you will need a reliable digital thermometer. Now it is a bit colder I also have a ceramic heat emitter on a thermostat to ensure the night time temps don't drop too low.
A large shallow water bowl should be in there too so they can soak or have a drink when they want. And its a good idea to use a piece of slate or something similar to feed your tortoise n to prevent them ingesting substrate. Also having cuttlefish bone in there is a good idea as they can get added calcium from it when they want and it will help keep their beaks down.