Information backup

I have a new computer. Upon startup, it created a D drive supposedly for the purpose of a backup drive. Now that I have used the computer for a while, I keep getting a message telling me that this drive is running out of space. I am also getting a backup warning that tells me that I haven't backed up the computer in over 30 days. When I try to back up the computer, it fails only to tell me that there is not enough space on the d drive to do the backup. How do I get off of this merry go round.