You can try folling links:
Question seems to be broad. but you can read this.
Welcome to Android Design,
Is there a standard layout for an Android app or should I design mine from first principles?,
10 Tips For Android UI Design,
for 9-patch images you can see this post by me.
Utility of android nine patch
Education Guidelines
For supporting multipal device layout based,
please go through Supporting Multiple Screens this link:
For multpal layout component
res/layout/my_layout.xml // normal screen size ("default")
res/layout-small/my_layout.xml // small screen size
res/layout-large/my_layout.xml // large screen size
res/layout-xlarge/my_layout.xml // Hd tablet
for orientation.
res/layout/ # default (portrait)
main.xml
res/layout-land/ # landscape
main.xml
res/layout-large/ # large (portrait)
main.xml
res/layout-large-land/ # large landscape
main.xml
1. You can also use `qualifier` for having multipal resource.