For alteration it is best to choose a famous work of art – so a new version will sound more bright and comical and, besides, you'll be able to sing along. If you don't want to redo the whole song, select the smash-hit, the content is the closest to the theme of your celebration. For example, if a holiday on which you will play new year, give preference to the song, which is sung about the New year.
As a rule, the text of a musical work written in the first person. Altering the male song to the female, remember that you are surely going to disrupt the rhyme, and this song in your performance will sound not as good as it could. Thus, if you are a "girl", choose a song where she sings (then the phrase "he loves me" "he kissed me", etc. will sound appropriate, and will not cause a smirk), and if you - "the boy" and sing it on his face.
If "your" song is already invented and even written by someone, just replace the key names from the author's text for the names of those present at the celebration of people (of course that was in a certain sense).
If overall finished piece of music suits you, but there are a number of nuances that you need to change, try to new words did not spoil the rhyme. Count the syllables, if necessary, tapping rhyme with a pencil on a hard surface or your own hand on the leg.
At the same time, nothing bad will that you will go beyond the source text. For example, if you transform a banal birthday song Happy Birthday to You, and the name of the birthday boy is quite long (or your team decided to address each other by name and patronymic), don't be sad if "cut" is nothing. As often happens, but, alas, nothing helped.
Remember that your song will be especially interesting for the audience if they hear mention of them. Think in advance what you could tell about someone, but at the same time not overdo it, and you heroes your song can hold the artist to the offense.