Dancing Ranger: all of the above is good advice, and I'd add a few things that have worked for me. But first, it's worth getting the joint checked, because 'arthritis' can have many causes ranging from infections to genetics to poor posture to old injuries and then some.
It's something with which I have *lots* of experience, since I'm pretty crippled up with 'arthritis' myself. I had rheumatic fever as a little kid, no antibiotics in general use back then, so that left some permanent damage to joints and heart. I then proceeded to live a very physically vigorous life, which left me with quite a few old injuries and subsequent osteoarthritis, which is the kind you get from wear and tear and overuse. Then the family genetic tendency to rheumatoid arthritis surfaced and added its note to the symphony of discomforts. Worst part is that the osteo is better for warmth, the rheumatoid for cold weather, so there's really no comfortable time of year.
However, advice:
. Get it checked out, since treatment options can be very different depending on cause.
. Keep moving. If you stop physical activity you stiffen. Swim, stretch, walk, do stuff like tai chi or gentle yoga (I do my old martial-arts exercises, but slowly). Try to work out your own best balance between rest and exercise, it's different for everyone, then work within your boundaries, maybe stretch them gently.
. Check for triggers. Heat, cold, humidity, particular foods, postures (standing for a long time on a concrete or otherwise cold floor is a really common one, to the point where there's a term for it - process workers' arthritis).
. See an exercise physio. They can help a lot.
. Try support for the joint, straps or bandages or tape. I found one of those neoprene sleeves worked well on my crippled-up left hand, gave me back a lot more use of the fingers and eased the pain.
. Some people get good results from liniments. I use Flexall or Tiger Balm on my knees, and it helps.
