There's a lot of trash that needs to be cleaned up.
Literal
Clean-up must lots-of trash [subject-が] exists.
The relative clause 片付けなければならない ('must clean up') modifies たくさんのゴミ — 'a lot of trash that must be cleaned up.' The whole structure feeds into an existential sentence with がある. ゴミ in katakana is the standard spelling for garbage/trash.