How Can I Get HLA Typed To See If I’m A Bone Marrow Donor Match For Someone, And Do I Have To Stay On The Donor Registry If I’m Not Compatible?

If I know someone who will be needing a bone marrow transplant and I want to know if I’m an eligible donor for them how would I get HLA Typed? Do I need to apply to be a donor before I can get tested? If I get tested and I’m not compatible with them will I need to be okay with still donating for someone else’s transplant or could I take my name off of the list?