DRDS93, you CAN replace just the 'D' string, but if you have been playing for 2 months with those strings, I'd recommend that you replace all of them at the same time, and clean the fretboards, etc., of your guitar, while you have all the strings off. You need to replace your strings fairly frequently, if you are playing it a lot - not sure what the recommended time frame is, but I pick up two sets and some extra G, B and E strings (they seem to be the first to go, especially when you are first tuning them, or if you muck up, like I did!) There is a section here on this website on how to go about replacing your strings, and I suggest you read it through, as I didn't, and made some minor mistakes which proved to be major, in that my strings did not last nor stay tuned.
http://www.guitarforbeginners.com/restring.html
Also, have replacement 'pins' ready to go just in case, as well. If you go to your local music store, let them know what type of guitar you have and they will recommend a specific set of strings for you (you need to keep the gauge about the same so the tension in the neck of the guitar doesn't change).
Hope this helps you!
Elle