Need help with basics

[Charming Charles]

Just starting to write addons, thanks for your help in advance!

I need some simple examples to start learning, trying to show a solid color box in the center of screen when certain event happens. In particular for the following few conditions:

1. The third (for example) rune of death knight is ready to use.
2. The player (myself) gains a particular buff.
3. Enemy target has a certain debuff cast by myself.
4. If a spell with CD is ready to use.

I know there are existing addons for these things. But I really want to write my own codes! Digging into other's codes of thousands of lines is not fun either...Again, thanks guys!