If you go to a pet shop, they have cage kits with everything you need except food and bedding. You need a cage, water bottle, food dish, bedding, food, chew sticks, and a wheel. The wheel is for them to get their exercise. With hamsters it does not matter which type wheel you get but gerbils require the solid type, not wire. The best type of bedding to get is the paper type. It is made from recycled paper. The cedar bedding will make them terribly sick, do not get it. The oils are poisonous to them. Toilet paper or paper towel tubes are greatly loved by them. They will chew them apart and it will be added to the bedding.
Mine always just nestled into the bedding and did not use shelters. Their teeth continue to grow for their life and so the chew sticks are to help keep them from getting too long.
Anything else you get for them is not basic, it is for fun and their or your enjoyment.