Depending on how your arthritis affects you, you may need to change many of your working habits. For example using a telephone headset is a good idea for someone with arthritis; it prevents neck strain and means you have your hands free, which is especially important when you find it difficult to hold things. If typing is becoming a strain, why not get your company to invest in voice-command software instead. Using a mouse can be a problem for many people so try using a touch pad instead. There is also a variety of ergonomic keyboards and wrist rests available so chance are you can find something that is comfortable for you.
If you find sitting uncomfortable, you may need a new chair with good back support and armrests or you just may need to adjust the way you sit. Make sure your feet are firmly on the ground and keep your back straight while working. You don't want to be leaning forward all the time and putting a strain on your back. Another idea is to get a sit-stand workstation so you can alter your position as necessary.
Look at what sort of ergonomic tools or aids are available. A simple document holder attached at eye level to your computer screen can make all the difference. Also keep your files and folders within easy reach so you don't have to do any painful bending. Another solution is to get electric equipment, like staplers, to make your life easier. You may want to consider adjustable, ergonomically designed worktables, grip pads, or other aids for arthritis sufferers. Also make sure that you take short breaks every hour or so and step away from your desk. A little relaxation exercise, or moving around to limber up, can prevent joints seizing up and help you to keep pain at bay.