Hi Toni, is it becuase you sound out of key? try using a capo on different fret possitions.
mercer i believe that would only confiuse the issue in my honest opinion..
i`d have to agree with papaguitar,,good sound advise
Tonyespo
judging by your post i assume that you are not playing all that long so without doubt i`d definately say all that you suffer from is lack of experience,,it really is as simple as that my friend..ive been playing quite a while now and at the minuite im teaching a friend of mine who has the same problem in fact i had the same problem myself when i first started to sing while playing and that was many years ago..here`s a tip PRACTICE !!! PRACTISE !!! PRACTISE !!!..there is absolutely NO substuitute for it..
what you also have to be careful of is,try not to pick a song that is (how shall i say it) ermmm a bit akward to play especially if your learning..try to chose a song that you are comfortable with singing and most importantly KNOW IT and KNOW IT WELL ! then add the chords to the song slowly as you progress ,,you be amazed at the amount of guitar players that i meet that have a fair amount of talent YET can barely play more than 2 or 3 songs ALL THE WAY THROUGH..If your familiar with playing songs in different keys firstly learn the song in the original key so that you know the chords then pick a key that suits your voice and try learning it it that key.
Make good use of Youtube there are lots of decent guys on there who may already be playing the song/songs that you like were you could gain some inspiration and tips on how they are playing it..All in all have fun with it
hope this is of some help