Semi, the problem with using well-known tunes is that people know them and will immediately compare your lyrics with the originals or, in their minds, sing along with the originals. I really don't think you'll be doing your song a favour by doing this; much better if you come up with an original tune of your own (give it a try - you may be pleasantly surprised with the result), of find someone who can write a tune for you. The satisfaction of having a totally original song to perform will be immense.

Failing that, search around for a more obscure tune and use that.