easypeasy, Lily . . . .as a lot of the posted replies have pointed out, just try any tune you have never seen written down anywhere (hopefully there will be some such as "Happy Birthday" that you have never seen in notation form) and play it . . .that simple to start with . . .
last night I picked up a dulcimer and made up another tune . . . so you could start with a scale and bend it into a melody . . . enjoy!
(If music is not enjoyable, stop playing immediately and eat icecream! . . .)
ps. . .I cannot read a note of written music . . .but I play lots of music!