。
I look forward to playing baseball.
Literal
Baseball [obj-を] do-[nom-こと][subj-が] looking-forward [copula-です].
楽しみ ('pleasure, enjoyment, looking forward to') as a noun is the nominalized form of the adjective 楽しい. Combined with ~がです, the construction means 'X is (my) enjoyment,' which idiomatically translates 'looking forward to X.' The こと nominalizes 野球をする.