Here is the melody of “Have yourself a merry little Christmas” tabbed out for guitar.
first version was missing major parts of the melody, so I tabbed out my own version.
to the song to get the rhythm. This is my 1st tab, so feel free to share any 
criticism in the comments. Enjoy the tab, and Merry Christmas!

e------------------------------------------------------------------------------|
b------------------------------------------------------------------------------|
g---------12---------------------------12-------------12-----------------------|
d----9-12----12-10-9--------------9-12----12-----9-12----12-10-----------------|
a-10-----------------12-10-12--10-------------10----------------12-10-9-7-5----|
e------------------------------------------------------------------------------|
“Have yourself a merry little Christmas. Let your heart be light…”


e------------------------------------------------------------------------------|
b----------------------------------------------------------12-10---------------|
g---------12---------------------------12---------------12-------12-11-9-11-12-|
d----9-12----12-10-9--------------9-12----12---------9-------------------------|
a-10-----------------12-10-12--10--------------10-12---------------------------|
e------------------------------------------------------------------------------|
“Have yourself a merry little Christmas. Make the Yuletide gay…”


e------------------------------------------------------------------------------|
b------------------------------------------------------------------------------|
g------------------------------------------------------------------------------|
d-10-10-10-9-7-5-7-9--9-7-5-7-9-7-7--7-7-7-5-4-2-4-5--5-7-9-10-12--0-5---------|
a------------------------------------------------------------------------------|
e------------------------------------------------------------------------------|
“Here we are as in olden days…”

e------------------------------------------------------------------------------|
b---------------------------------------------------------12-10----------------|
g---------12---------------------------12--------------12-------12-11-9-11-12--|
d----9-12----12-10-9--------------9-12----12--------12-------------------------|
a-10-----------------12-10-12--10-------------10-12----------------------------|
e------------------------------------------------------------------------------|
“Through the years, we all will be together…”

e--------------------------------|
b-12-12---------12-10-8-7---7-8--|
g-------9-12-12-----------9------|
d--------------------------------|
a--------------------------------|
e--------------------------------|
“And have yourself a merry little Christmas now.”

Lyrics:

Have yourself a merry little Christmas,
Let your heart be light
From now on,
our troubles will be out of sight

Have yourself a merry little Christmas,
Make the Yule-tide gay,
From now on,
our troubles will be miles away.

Here we are as in olden days,
Happy golden days of yore.
Faithful friends who are dear to us
Gather near to us once more.

Through the years
We all will be together,
If the Fates allow
Hang a shining star upon the highest bough.
And have yourself A merry little Christmas now.