How should I paint the knotty pine paneling in my den?

You can "white wash" it with watered down paint similar to dry brushing. This will lighten it up but still leave the grain. To paint it you would need to lightly sand to remove any gloss, prime with a stain blocking primer like Zinsser123, then paint with your color.

Bill is right. But a simple coat will not cover the old ugly yellow pine if it has glossy finish, which most do. you will need to sand and need to kilz and paint a few coats. Been there, done that, long chore, but amazing results and high price home improvement value. No one wants to ever see those glossy yellow pine walls again. Use a butter cream color satin finish paint. Repaint your ceiing white.

I suggest using chalkpaint- it adheres to most surfaces(including wood) and it gives a really nice, vintage look!

https://www.pinterest.com/explore/knotty-pine-rooms/

I agree with Hillela....Chalk paint will work really well! You don't have to sand or prime, AND you can use it indoors!