Practice building, experiment, try scripts, textures.
Do all this and more in Effex! The region is available to visitors - build is enabled. Just clean up before you go. If you think others will be interested in your creation, feel free to leave it there for a while.
Just a few simple rules:
* Be considerate - avoid using lots of prims
* Be more considerate - try stuff, but don't offend anyone
* Remove your things before you go or pick them up after a few days
Enjoy :)