How do i tighten serpentine belt .

Opposite of loosening.

12 & 14 mm. wrenches or sockets.Loosen 12mmbolt-head on back-side of alt. slide adj. bracket. Loosen 14mm bolt-head at pivot(base of alt ,by eng bracket) Turn adjuster bolt (the one going down into the slide part )turn clockwise to tighten belt , counter-clockwise to loosen. Even if it's a new belt as you say , if it's glazed ,replace it preferrably with toyota belt.