。
I'm planning to go skiing with my girlfriend.
Literal
I [topic-は] girlfriend [with-と] together-with skiing [to-に] go plan is.
スキーに行く ('to go [for] skiing') uses に to mark the purpose of the going — a special construction for 'go to do X.' 一緒に ('together') is partly redundant with と but adds warmth/emphasis. ~予定 ('plan, schedule') functions as a noun expressing future intention.