Some suggestions.
Start with this video -
DOTA 2 in 4 min- This will cover objectives and the general layout/mechanics of the map.
The often quoted -
Welcome to DOTA - You Suck guide should also be good prerequisite reading as well.
From there, more Purge stuff...specifically his learn about DOTA vids (for starters)
How to Spend your Starting Gold
Purge Gives an Item Summary- Goes through shop layout and items one by one
Purge Talks Trilane- You most likely won't ever trilane as a beginner but this video explains some ganking positions as well as proper areas to stack/pull during the laning phase (two things that you will be doing no matter what)
Purge Talks Lanes- How the laning phase works
and his Purge plays as (insert hero here) series on how to specifically play certain heroes.