Woodland Tower is in north Hyrule, northeast of Hyrule Castle. This page has advice for finding and activating it.
Reaching Woodland Tower
You can see Woodland Tower if you look northwest from Lanayru Tower. Warp to Lanayru Tower, and float northwest so that you land to the right of the river. Continue going northwest along the riverbank toward the tower that you can see in the distance.
Eventually you will see a shrine on the right, and a stable near that. After the stable, go right at the fork in the road. Then when the path splits again, go northwest.
From here, you can go to the Woodland Tower.
Climbing Woodland Tower
The base of the tower is a bog, but you can use Cryonis to create ice blocks and hop across them to the tower. You will need to climb the wooden platform on the north side of the tower if you want to reach the tower's lowest platforms.