It’s a prerequisite to attain a Visa that costs 50 USD by most people coming from various different countries, intending to go to Kenya.
Attainment of Visas is either directly from entry or in case of a Kenyan Diplomatic mission in your nation, a Visa is to be got in advance.
A Visa’s validity is up to 90 days.

To avoid any inconveniences whatsoever, see to it that your passport is valid for a minimum of six months, starting from the date of entry.
An onward or return ticket must also be presented by you or else you may require to make a refundable deposit or buy one.

You can apply for Visa extensions at Nyayo House in Nairobi or the Immigration Office in Mombasa to the Principal Immigration Officer. There is no need for you to get re-entry passes if you travel to Uganda or Tanzania provided that your visa is valid.

You will not be required to pay upon leaving the country as it was since the Airport taxes are now part of your flight ticket.