you should really record this with multiple tracks. This way, you can concentrate on playing the guitar, play it back, and sing to your own guitar playing. edit out background noise, and overlay the tracks together. This way, you can also add harmony...but I guess I'm getting ahead of myself.
another cool thing about layering recordings is, you can isolate individual sounds. For example, if anyone asks to remix the song, you can give them the stand alone vocals. That's just one idea I guess
as Pon-3 said, you get a bit off tune in some parts. easy way to not do the whole take again is to sing each verse as a separate section. and then splice them together.
a cheap USB microphone that give a good quality input is the Blue Snowball
http://www.bluemic.com/snowball/Alot of artists use it cause it's cheap and the sound quality is apparently great!
Best of luck to you, an as always: practice makes better!