I wrote a song but I don't know what notes my song contains... Is there any easy way to figure it out?

i wrote a song with my best friend, because we are starting a band for
the summer... but we know how to sing it we just don't know what notes
were singing... i also play drums, guitar, piano, clarinet, french
horn, trumpet, and saxophone, and i need to write an accompaniment
pattern for piano guitar that will sound good with my song, and i
would also like to write a score using the computer... so i am asking
for 3 things:
1. help on finding out the notes i'm singing easily
2. help on writing an accompaniment pattern that sounds good with my
song
3. a FREE software that i can use to write a score such as a free
version of pizzicato
if you could please help me with any or all of this that would be
perfect! thanks!
problem 1 is officially solved but if u think u have a better idea
have at it!