Hi Xing,
There are a couple of options if you are interested in classical music: As pianoluvr suggests, you can look for books with easier pieces by many of the main composers. IMO, it does get harder to make sure that you are working pieces in order from easy to hard.
If that is an approach that interests you, here are some of my suggestions:
JS Bach: Anna Magdelena Notebook then 18 Little Preludes.
Clementi: Sonatinas Opus 36
Mozart: An Introduction to his Keyboard Works.
Beethoven: An Introduction to his Piano Works.
Burgmuller: 25 Easy & Progressive Studies Opus 100
Gurlitt: Album for the Young Opus 140
Schumann: Album for the Young Opus 68 then Scenes from Childhood Opus 15
Streabbog: 12 Melodious Pieces Opus 63
Tchaikovsky: Album for the Young Opus 39
Bartok: Album for the Young
Kabelevsky: 30 Children's Pieces opus 27 & 24 Little Pieces Opus 39
Prokofieff: Music for Children Opus 65
Alternately, you can work out of books that are graded by level. Keith Snell is the series I am working out of. I'm not sure how much of a challenge it would be to get his series in Austrialia. Here's a link to a guide for his series:
http://www.kjos.com/pdf/brochures/snell_reper_index.pdf There are lots of other series set up in levels like Keith Snell's.
Hope this gives you some ideas.
Rich