There is definitely stuff on the web, but it's not something I've looked into myself.
As with a lot of instruments, what you might need to learn can depend on the style of music or accompaniment you want to play. Bleusgrass playes have a completely different style compared with people who play Irish reels. Plectrum work for sustaining a note behind a slow folk song is diferent from the way a country musician might back a slow country song. When you have a direction to follow you can ask for more specific advice on the chosen style and the techniques appropriate to it.
Basic learning includes chords - have you got a decent chord chart? You can never go wrong using common scales and arpeggios to familiarise yourself with the finger board and to train your fingers to find the notes you need without having to think about it.

As far as tunes are concerned on the web both www.concertina.net and JC's ABC Tunefinder (can't recall the URL) are good resources for finding music which can be turned into printed dots.

Quack!
Geoff the Duck.