Probably means a link to the posts you've seen.
And, yeah, as said above, 15mm is just a scale to which models are sculpted. Some games systems use it. Others, like
40k, use 28mm. Some use 32mm, like Malifaux.
As for how you get into sculpting... well, that's a whole thing and there others far better placed to instruct. Like with painting, though, there's loads of tutorials on the web.