Pine is absolutely horrble to try to stain! LOL
You will have dark boards and light boards, bright grain and dark..
The stain will wipe right down to bare wood in places and soak in deep and dark in others at random.
The best you can do is to use an orbital sander with 120 grit, clean it up as evenly as possible. Then apply a stain evenly with a brush, wiping down after it gets a little tacky to touch. The grain will pop out and the knots will glare at you, but that is the nature of pine.
Good luck brave soul!