。
I'm looking forward to seeing you next week.
Literal
Next-week honorific-meet-[nominalizer-の-を] enjoyment [into-に] doing-is.
楽しみにする ('to look forward to') is a fixed collocation — literally 'make into enjoyment.' ~ている makes it ongoing ('am looking forward to'). お会いする is the humble 'meet.' A polite, warm closing common in business correspondence.