why do you want to stick to just one string? Though its possible to transpose song into single string but its not advisable and actually of no use at all from learning purpose. Sooner you can start using the other strings better it will be.
But since you have started guitar just few days back, please do not jump directly to song. Initially it looks very attractive but its not gonna help in long run or put it in other words, you gonna take much longer to learn guitar.
I would advice you to go to Bignners's forum on this site and start with some finger practice. Do that atleast for a week regularly. Once your fingure becomes slightly relaxed wrt to guitar then probably you can think about taking simple songs.
Simple yet more powerful execercise is tabbing on first 4 frets with your 4 fingures from bottom to top and then from top to bottom.