First , please bring her in to let the vet assess the severity of the condition and see what the vet suggest.
Second, make the home comfortable by either lowering the places she use to climb or make steps so that she can get up easily.
One important place is the litter box. Make sure she can move in and out easily. Either place some step up to the litter box or get one with a real low opening so that she can get in and out without climbing.
These are some of the suggestions I can think off and maybe more members with cats of the similar experiences may have better ideas will chip in.
As tabbytom
mentioned, the first stop should be at the vet. An x-ray will show arthritis. Then, you can discuss with your vet some supplements like Cosequin. I give my senior girl Cosequin and it really keeps her limber and less gimpy. Provide, lots of soft, warm places to nap. Look into purchasing a heated bed or pet bed heating pads that adjust to the ambient temperature of the room when not in use and only heat up to a cat's normal body temp. I have ped bed inserts for the winter months for my kitties and they LOVE them, but my senior girl pretty much lives in that bed all winter long. For high places, provide steps or place a chair to use as a boost up to the higher spot.