Thanx to the excellent Make It In Music for pointing us an article on CMU about Ten Tips for New Bands.
And looking at the article, we would say that those tips seem obvious from a distance, but many bands, even already established, do not take enough attention to point 4: "Write an agreement between bandmates".
It all looks great when starting a band with your pals and, honestly, it doesn't feel like anything bad can happen but when money is involved (we wish you to make some when selling your work) EVERYTHING can happen.
So, yes, write down an agreement with your mates as to whom gets the credits : who wrote the song ? who wrote the lyrics ? What about putting down all the band members name as arrangements credits ?
Don't forget: there are several copyrights when commiting a song on "tape": composer/lyricist/arranger and who paid and owns the copyright on the sound recording (the producer, often the record company).
Make sure also that you identify clearly who owns the band's name. I know, it seems odd to want to be sure about that point (remember, for now you're just 4 friends playing in a rehearsal room and your conquest of the World hasn't...