I recently got a hobby sawmill(Woodmizer LT10). How do I cut a log so
wooodturners can use the wood?

Depends on the wood turner, if they do spindles or bowls. Regardless, you
will always want to saw out the center of the tree, called the kant. Can't turn
kants! For spindles, 2x2, 3x3, and 4x4s are needed. For bowls, slice off a
split, then cut down the center, then flip and remove the other split. cut the
blanks to length. The length longer than the width to allow for checking.

I assume ya'll would be interested in the
knottier sections, is that correct?

Depends on the wood turner.

If I simply cut a log into big squares, what do I do with them so they don't
crack?

You want to seal the end grain immediately with Anchor Seal.

Which kinds of wood are preferred? For the most part, the classic furniture
woods. Also, exotic woods.