When you consider the use that your current spare room gets, you will probably be in a similar situation to most other home owners. Your spare room is not serving much purpose, unless you have visitors staying over every weekend, changing your spare room into a home office can be a great change.
If you work is computer based, or you just want a separate space to browse the web or do some writing, a home office is the best option.
Creating a minimalistic area is often favour, as an open clear space provides you with a better environment to complete tasks. Stick with light neutral colours, and keep your wall plain, do not overcrowd your office with pictures as these cause distractions.
Selecting a desk and chair is your next priority, you need a good amount of space for you to do your work on and a chair that is comfortable, yet keeps you with good posture and support.