How do I fly higher? I want to see what's on top of this building.
To fly upward, you may hold Page Up or hold the E key (the Chat bar should be closed; otherwise you'll end up saying "EEEEE" a lot). Also, selecting View > Movement Controls from the menus at the top of the Second Life window opens controls that you may find helpful.
An unassisted avatar can fly to slightly less than 200m above ground level, and may fly higher with the help of a scripted object or vehicle. An avatar's maximum hover height, without input from you, is 50m plus the height of the terrain. If you are above this altitude and you stop trying to actively fly higher, you will sink to the maximum hover height.