I was browsing about Hashirama's sage mode. What I could procure was that he went to Shikkotsu woods to learn the wood style and the sage mode respective to that.
Shikkotsu Forest (湿骨林, Shikkotsurin, English TV: Shikkotsu Woods, Literally meaning: Damp Bone Forest) is one of the big three unexplored sage regions, a legendary place that is equally famous as the other two: Mount Myōboku and Ryūchi Cave. It is the home of the slug Katsuyu, parts of which are summoned by Tsunade and her disciple Sakura Haruno. - source
How did Hashirama come to know about this forest which is the home of slugs? And what was his background to learning the Senjutsu? Additional question that arises is that Does this mean both Tsunade and Sakura can learn the sage Jutsu?