deciding the topic/niche?

[smiling]

It is normally the low paying keywords that make money for my site ... Don't necessarily look for big payouts but be happy with 10 cents clicks.
Develop something you can write about for a long time . The ads and money will come in slowly as traffic rises.

G is a long term play.. ( as many senior members will tell you ) . Don't really expect huge income in a short time. If you do that , you are likely to cut corners and then the problems start..

Build test pages on different subjects, see what works and what doesn't work.