Which side of the building would I put them on and what part of the roof? The apex?

It’s not that easy. Generally the placement maximizes sun exposure while minimizing hazards, threats, and expense:
* South facing in the northern hemisphere, angled upward from level approximately by the installation’s latitude. At the equator, they can lay flat. At the north pole, vertical.
* Away from chimneys, flue vents, overhanging trees, etc.
* Minimize strong wind exposure—or at least decrease deleterious effects of strong winds being able to uplift or remove the panels.
* Reasonably convenient maintenance access

They need not sit directly on the roof—called "flush mount". Usually either a lattice-style frame or a tower is mounted to the roof or ground and the panels are mounted on it. See these examples:
* http://www.wdtrade.com/showroom/tradelead/1073207/Swimming_Pool_Solar_Heater_Compact_Solar_Heaters_Solar_Energy_Power.aspx
* http://www.solar-electric.com/PV_Mounts/Solar_Panel_Mounts.htm
* http://www.solazone.com.au/frames.htm
* http://www.wirefreedirect.com/solar_panel_pv_mounting_frames_and_accessories.asp

Also there are "tracker" systems which face the panel toward the sun based on the time of day and day of year.