I need some guidance on Lithium Framework:
Thank you in advance!
Lithium has a cli tool. It's called 'li3' and you can find it under the lithium/console directory. It creates models, views, tests, and what not. As for naming your models and tables, plural names are the convention. Posts, Tags, and PostTags as you mentioned -- although there is no HABTM support in li3.
You should checkout the manual available at http://li3.me/docs/manual. It's official documentation source, and is based off the more up-to-date li3 manual repository at https://github.com/UnionOfRAD/manual.