Knee joints issue. Cap. please advise!

I have been suffering from knee pain from last one year and it started from the time I joined gym. The knee joints are always stif and I and tight and slowly increasing day by day. Visited couple of doctors where I was asked to do couple of exercises without getting any proper resolution and was told that it was nothing severe. The stiffness around the knee joints are consistently there and last doctor told me that it's osteoarthritis and he asked not to do any exercises. I seriously have no clue what's happening and quite worried thinking I have to be with this for rest of my life. Not even a single doctor could give me a proper solution. Please suggest me something as I am really worried and looking for some permanent solution. Please also advise whether I can continue yoga classeswearimv my knee cap. Please advise!!