What should a man in his early 20's do to prevent from losing his hair?

A little: Male pattern hair loss is pretty hard to stop. Some medicines, such as Finasteride (in a topical solution that you apply to the scalp or pills) can help slow down the loss. But i suggest you try to view hair loss as a natural and not unattractive process. After all, it is uniquely male (usually) and many if not most women find it attractive.