Boot Camp Assistant problems

I am trying to install Windows 10 on my 2015 MacBook Pro 13.3" running Mac OS X El Capitan (10.11.2) but I am unsuccessful (for software that is not supported by Mac OS X). I have tried several times but it always says that my disk space is not enough (cannot remember the exact words in the error message). I then opened the Boot Camp Assistant through the terminal and saw that it only recognizes that the drive would have 4GB of free space available (I have a 500 GB SSD and 130 GB is used) but some insane amount of about 140000000000000 GB was used (what?). When I check in the Disk Utility I also get some suspicions that Boot Camp Assistant selects the Macintosh HD partition, instead the the actual drive. In the Boot Camp Assistant I can choose the size of the partition (I want to leave 370 GB for OS X in the Macintosh HD partition and get 128 GB for the Boot Camp / Windows 10 partition). I can't get this to work and I am grateful for any help!