www.musictheory.net is a great resource for learning guitar.
This is a great free resource on the web. You can set the number of guitar frets you want to concentrate on. Let’s say you are trying to learn the notes on the first three frets of the guitar. It then flashes up a position and you have to say what note it is. Have a go if you have got a spare 5 mins. There are lots of other things to try out too on this site.