They are very simples, If you want to apply for a Visa On Arrival that lasts for 30 days then you should be one of the eligible countries(check the link), have a valid passport with at least 6 months remaining and pay 25$ USD entry fee in cash. As for the E-Visa for 30 days, you should have a valid passport for at least 8 months, complete the online application, pay the e-visa fee then print the e-visa to later be presented to the airport border guard. You could be one of the countries eligible for a free visa for 90 days.