Normally you as the freeholder can rent out your condo apartment in Thailand, unless the rules and regulations of the condominium regulate or restrict the right of the owners to rent out their unit.
A lease or rental contract in Thailand should be in writing and any lease rent with a fixed term over 3 years should be registered with the land department (section 538 civil code) and includes a registration fee of 1.1% over the total lease price. Translation of tenant and landlord laws are included in this website here.
Rental as a business
If you would rent out your condo as a rental or property business then you would need a work permit under the foreign employment act, and you would need to get a foreign business license under the foreign business act, or work under a Thai company. Rental business is restricted for foreigners under list 3 of the foreign business act.
Short term rentals
Short term rentals (less than 30-days) may be subject to the Hotel Act, therefore need compliance with the Hotel Act, even if an exemption applies to a single owner.
Taxes income and property tax
Income received by a foreigner in Thailand, from Thai sources, like the rent received from renting out your conde, is officially subject to income tax pursuant to section 40 Revenue Code. In addition the rent received by the owner from renting out his condo (or other real estate) is subject to a 12.5 % property tax of the yearly rent (under the land and building tax), payable to the local administration before the end of February each year.