What can I use to stop losing hair?

Depends on cause. For most common kind, androgenetic alopecia, can try over the counter product called rogaine, (minoxidil) or a prescription rx called propecia, each of which can be partly helpful, but are unlikely to result in full restoration of hair. There are cosmetic procedures (ex. Hair transplant) that you could talk with a dermatologist about. Wigs & hats are an option. Or you could just "go with it". C ur doc 4 eval.