I need to learn basics of STL programming? Where and how to start?

I don't think that here is any stl covered. If i would be you and it is Siemens specific i would start at the manual
https://cache.industry.siemens.com/dl/files/814/109751814/att_933093/v1/STEP_7_-_Statement_List_for_S7-300_and_S7-400.pdf

i also can recommend you the STL cheat sheets.
http://www.plcdev.com/files/plcdev/STL-cheat-sheet-by-alphabet.pdf
http://www.plcdev.com/files/plcdev/STL-cheat-sheet-by-category.pdf

i'm bumbing this thread. There are no complet/good courses on the internet for STL language. Maybe you should consider this. I have several programs in this language...Thank you