If you are currently a runaway, then the people you are staying with are potentially breaking the law by harboring you and they could be arrested and go to jail. Unless they take the appropriate steps to contact the authorities, there is not a chance on Earth they will be able to gain any form of temporary custody of you.
To get into foster care, the court (generally through action by CPS) has to declare they have jurisdiction over you. They then can assign placement. It could be with a friend, but will usually be with an established foster care program or local family member. But, if CPS and the court are not willing to remove you from the home (likely for lack of substantiated abuse) then no foster placement is forthcoming.
- Carl