Phrases like Worship Service or Service of Worship are tautologies. To worship God means to serve him. Basically there are two ways to do it. One way is to do things for Him that He needs to have done - run errands for Him, carry messages for Him, fight on His side, feed His lambs, and so on. The other way is to do things for Him that you need to do - sing songs for Him, tell Him what's on your mind and in your heart, in general rejoice in Him.