There are many different websites that accept classified advertising, either as their sole method or as part of their system which can include forums or news type information.
One of the most common websites for posting classified ads in your area is Craigslist.
First of all you will need to set up your account with them.
There are several reasons to have an account, the most important of which is that then all your previous ads will be available for you to look at or even repost with changes or as is, depending on how successful it was.
They have certain rules that you must follow at the risk of getting flagged for spam if you overpost, so do read through their terms of service to make sure you understand what not to do.
They are particular about duplicate posts in different areas, and prefer that you post only to your local area.
They insist on putting your postal code in new posts, which has changed from their previous rule.
See more about how to post a Craigslist ad here.
Fill in all the appropriate spots - use some keywords in your post title, because this is what people will see.
Also include a picture or several, as that also will encourage people to click through. Make sure you have some way for prospective customers to contact you, and keep it friendly and professional.