If hes never lunged before, start by walking him round in hand so he gets used to being in an enclosed space. Then stand in the middle and have someone else at his head. Gradually reduce the influence of the person at the horses head. Eventually he'll be lunging! For trot, ask like you normally would on the lunge. If he doesn't understand, reintroduce the handler until he gets it.
Lunging isn't always neccissary, but it is useful for teaching horses to properly carry and support themselves. What are you wanting to do with him? If you need to do schooling and hes tense in the school under saddle, try schooling him in a field or out on the trails.
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz