The ideal setup is a desk with a keyboard tray (so the keyboard sits lower than the surface of the desk), and a proper office chair with a gas lift and a low back, which you can adjust to sit into your kidneys.
If you can't afford that, there are a couple of alternatives.
If you can get a desk with a keyboard tray, buy yourself a blow-up fitness ball and use that as your chair. Choose a size that puts your forearms at a 90 degree angle to your elbows, when your hands are sitting on the keyboard.
If you can't get a desk with a keyboard tray, buy a plain table instead and put your money into a proper office chair.