If the weather's nice tomorrow, I'm planning to go on a picnic.
Literal
Tomorrow, weather [subject-が] good [if-ば] picnic [to-に] go intend [polite-です].
天気がよければ uses the ~ば conditional on the い-adjective いい (via irregular よければ form — いい/よい's conditional is よければ, not いければ). ピクニック is a loanword. つもりです expresses the speaker's intention contingent on the condition.