Every year when spring comes, I look forward to going out to the garden.

Literal

Every-year spring [into-に] become-[when-と], garden [to-に] go-out thing [object-を] looking-forward [stative-にしている].

春になる ('become spring') is the idiomatic 'spring arrives' — another state-change expression. ~と is the temporal-conditional 'whenever X, then Y.' 楽しみにしている ('is looking forward to') is a set stative construction — 楽しみ ('anticipation') + にしている.