I know you've probably searched the web for sites and such, but I found this site that gives you idioms, what they mean and an example...you can browse through them and pick a few.
http://www.english-idioms.net/wm/main.cgi 1. Yeah, I probably do use idioms occasionally, but I don't really notice when I'm saying them. It's just sort of natural. You hear your parents say a few and suddenly you're saying them all the time. Like, my dad says "Six of one, half a dozen of the other" ALL THE TIME. (http://www.english-idioms.net/cgi-bin/idiom.cgi?idiom=six%20of%20one%2c%20half%20a%20dozen%20of%20the%20other) And then there are things like "the grass is always greener", or "too many cooks spoil the broth" They're all the sorts of things we randomly say around my house...it's so strange, it just comes out.
2. I think my faves are "Kill two birds with one stone" , "Don't look a gift horse in the mouth",
3. I have no idea. I know sayings and such have to come from somewhere, but no-one ever explains it. You pick them up as kids and then suddenly you know them.
4. I can't really think of any others like that, except maybe "shut up"/"get out", used in a a "no way" type ....eg. "You got the new CD? Shup up!" ... Think
Mean Girls movie/American teenager movies.
I hope that's helped a bit. The website I linked above really does have a good list of them. Unfortunately there are a LOT of idioms out there. I don't know how many you need.
by the way, I'm in Australia.