How can I become a caregiver?

What's the issue between her mother and legal guardian? Was the teenager removed from her mother's custody? I would think the guardian would be the one to give permission, and I'm also assuming that at 17 she's not yet considered "of age".

You might want to contact the juvenile court or the court that issued the guardianship orders to make sure everything is done properly.

Don't forget to address support (i.e., financial issues).

Trying to help here but I don't have much experience. Is she still in school? Is there an organization that helps homeless families that might point you in the right direction? Or the division of youth and family services?
To protect yourself, I would look for guidance from one of these (or similar). Is the 17 year old in foster care? Family care? I would explore all options to be sure your friend gets all of the help she needs and that you don't cross a line that you didn't know existed.

Oh sorry this was the only website that i found that I felt can help, didn't know it was for family members only. Yes it is more of a housing situation. Is there anywhere i can go to find help about this subject?

Tanaka, this website is for caregivers who are taking care of elderly parents/relatives/spouses. Thus, we are limited on how to help when it comes to minors.

This sounds more like a housing situation then a health caregiving issue. Correct? Since you mentioned the minor will be finding a new place to live when she becomes 18.