Something is true if it accords with reality. In other words a true statement is one which matches the reality it represents. "1 + 1 = 2" is a true statement. When it comes to God, God is the Ultimate Reality and thus the basis for all reality. Reality relies on God for it's existence. Since God is true to himself he is true.
Everything good comes from God. God is goodness itself. Everything God created is good because God is good. God never intended evil to happen, but since He created free will, He allowed it to happen. God is so good he can bring good from evil. But evil always remains evil.
“God is infinitely good and all his works are good.” (CCC 385)