need help creating "send message" form

hi, i am creating a social network site for a class for graduation and i need a little help in creating a script that will allow users send messages to each other. i want them to send massages to each other on the site not by email. but i am stumped, i have no idea how to do that part of the site. i have created the registration and login parts, member area and the like. but i just cant get one user to send a message to another(i havent even figured out how to allow one user add another to his/her friends list). i will provide a link to my website later but i am currently uploading the files from my pc to the site. with this somewhat limited information, can anyone create the script or at least tell me the steps i need to take to accomplish this. Thanks
p.s.
sorry if i am just rambling, i havent slept for a day.