First of all, verify that your phone is a "BMW Approved" phone. Go to BMW's website. If it is, navigate the I-drive to communications and enable bluetooth.
Then go to "Tools" in your phone and search for the bluetooth connection with the car. Follow instructions. Depending on your car model. it may ask for a code. Newer cars only need any 4 digits (like 0000). In some, you have to connect a BMW GT1 (scanner) to get the code from the TCU (telecommunications control module).
Sometimes, it is also difficult to pair a phone due to the numerous old pairings. Delete the old pairings and retry.