If the weather's good tomorrow, I'll go sailing with friends.

Literal

Tomorrow weather [subject-が] good-if, friend [with-と] yacht-riding [purpose-に] go.

良ければ is the ~ば conditional of i-adjective 良い. ヨット乗り ('yacht-riding') is a compound noun from the loanword ヨット + 乗り (masu-stem of 乗る). The purpose-marking に connects the activity to the motion verb 行く.