Heya Dany..

If you're serious about learning how to play, take the whole guitar to a shop and have it "set up" with some new strings. This way, if anything breaks while it is being restrung, the guitar shop should be liable and will have to fix it.

If you just want to play around and you don't care too much.. sure buy one string.. but you'll probably find that the other strings don't stay in tune and probably will also break soon enough too.