Most people that work from home use at least one computer, a telephone, and need a writing surface. Unless you already have a work desk most people have a kitchen/dining table. This gives you a flat surface and typically has plenty of space to spread your paperwork out. Dining chairs usually have a back rest and so is a good place to start. 

However, dining chairs are not designed for sitting for long periods so remember get up and move around. 

If you have a separate keyboard and mouse and can plug it into your laptop that is even better. Elevate your laptop onto a pile of books/small box so that you look more directly ahead - click here for more details.  Any elevation of the screen is better and avoid looking down at the screen for prolonged periods if possible.  

On the phone for a while and have the option, switch to the phone speaker mode so you do not have to hold the phone regularly. Even better get up and walk around if your work task only needs you to talk.