I've got an early stage of hair loss. What can be done to halt it?

More than before. If it's a genetic pattern of hair loss, minoxidil, Finasteride (prescription) and laser therapy (lasercap, laser brushes) can help, and the sooner you start the better. See a specialist in genetic hair loss to discuss the options. If it's caused by another condition, you need to see a doctor to get the right diagnosis and get treated.
Depends... ...On what is causing it. A dermatologist will run some tests to (hopefully) determine what is causing your hair to thin, and then treat you based on the results. Until you can get to your dermatologist, the only otc product proven to work is rogaine (minoxidil).