You'll want the for line to read:
for i in `find $DIR -type f`; do
You don't need to use the asterisk with find - you just give it the directory to search in. Note that, by default, this will be recursive and find every file in that directory.
You can run the find command outside of the script to see what files it will return.
As far as bash tutorials, I pretty much just search for "bash tutorial" on Google whenever I need to check something. There's not a particular resource to use, except maybe for the bash manpage