It's also possible that it's not serious. For example, I also started recently and starting out had pain in my knees. I bought two knee braces and hit the pool doing kicking drills 4 days a week for a month. (I also started carrying my board up the lift so it didn't hang on my knees and so I didn't twist them accidentally getting off) It made a HUGE difference. Now, 3 months later, I can snowboard for 2 days with no knee pain and no braces (definitely want to ween off the braces, as they can keep your knees weak, but it's great for while you are building leg strength) Also, it's much easier to learn how to snowboard first and then get off a lift with the board on, as it comes automatically with none of those horrible lift crashes
Keep in mind that you can't really strengthen the knee, just the muscles attached to it. Focus on working those and fully stretch out before and after boarding.