Either of those are correct. In the army you have to wear a uniform. In the army you must wear a uniform.

Probably "have to" would be most accurate. It implies that it is a rule and that the order comes from above you and you have no choice. "Must" is logical deduction or personal circumstance that makes something necessary. See article.: http://www.englishgrammarsecrets.com/musthaveto/menu.php Hope that helps.

"In the army you have to wear a uniform." Stephanie's link explains it well. Take note of the "personal circumstance / external circumstance" guideline and this will solve the dilemma.