I remember years ago when I first started playing guitar how
I used to hate it when I couldn't sing a song in the same key
as the original artist. I had to get my Dad to transpose them
for me on the guitar. That was until I learnt am easy way of
transposing songs. It is very easy if you can remember your
chromatic scale.. 

Ok, to start of, you write down your chromatic scale, staring
with the key for the song you want to do, I will use the key
of D.

  D D# E F F# G G# A A# B C C# 

ok, now you write ( or type ) the chromatic scale for the key
you want to change the song to underneath, making sure you 
start with the corresponding notes, eg the key you want to 
change the song to is F#, you need to match the F# under the
D.

  D  D# E  F F# G G# A  A# B  C C#
  F# G  G# A A# B C  C# D  D# E F

Now any chord that is in the song, for example D C G, D would
then be F#, and is directly underneath the D, C would then
be E, which is again directly under the C, and G would then
become B. If there is a minor chord, then you just add the
minor to the chord, eg, F#m, you take off the m and it is 
just F#, so under F# is A#, so then you can add the m again 
to make it A#m, the same applies for any other chord, such as
7, sus, dim, aug etc.. 
I hope this lesson is helpful to you...

Happy guitaring