let go=to stop holding the basketball.
Don't let the ball go sideway.
Don't let go of the ball wrongly.
Let go ! You are hurting me ! You are a thief !
Or to give up an idea or an attitude,or control of something:-
It's time to let the past go.
Come on,Lakers, let yourself go !
Let's go is short form of "let us"
eg:-Let us go for dinner.
eg:-Let us break up for the time being!