If you are the owner of an apartment, there are rental sites on line that will allow you to advertise, Trulia/Zillow both have this service but I'm not sure what the cost is.Of course there is also Craigslist and if you hire a realtor, they can advertise for you. If you've never been a landlord before, it may pay to discuss this with a realtor since there can be legal issues. E.g. Does your property insurance cover renters, how do you check potential renters financial stability, what are the renter's rights that you need to be aware of and what are your rights as a landlord, just to mention a few.
If you are a current renter looking to sublet your apartment, you need to check your rental contract and see if it allows you to do so.Many do not. I hope this was helpful.