Buildings - wide lenses. Distortion will really be noticeable because of all the straight lines. Just be conscious of it, you can use it to your advantage. Alternatively you can get software that corrects the distortions, but often you'll need to crop the image down a bit so make sure you leave a bit of a border so nothing of interest gets cropped. You'll see why tilt-shift lenses are used for architecture/buildings.
Same rules apply. Rule of third, leading lines etc. These are photography principles, not just landscape etc. You'll find lots of leading lines with buildings.
Probably try portrait orientation for tall buildings? Not sure.
People - man I really need to practise this myself. I'm no expert at it. Lighting very, very important. Pose too. Focus on the eyes. Still, rules of thirds typically, make sure they're facing into the frame, unless there's something compositionally beneficial for them facing out (eg using negative space for effect).
Useful tips for posing:
http://lumitouch.com/benstudiotutorial/index.html
http://jzportraits.home.att.net/
Hope that all makes sense.