My armpits sweat really bad. What clothes can I wear so that it won't be visible, and what can I do?

Topical medication. Type of clothes will not matter when there is an excessive anount of sweating. Generally this problem is not associated with an illness. There is a product called drysol solution 20 % to apply to armpits. This is a prescription so it can be gotten from your doctor.
Drionic, botox. Drisol is the same aluminum chloride found in antiperspirants and may work. However if ot not effective bottom can be used safely for underarm sweating and can be covered by insurance with prior authorization. Drionic is a safe low level current that has been used for 40 years without side effects.