You mentioned being confused by the use of .tcl pages; while it won't necessarily help you learn the OpenACS API per se, bear in mind AOLServer has ADPs, which allow mixing of code and HTML, just as you're used to with PHP.
Once you've started to get a feel for which procs, eg, check authentication or unroll news items from the DB, you can start using them in an ADP framework if that makes you more comfortable.
