There are many web classified sites where you can place your free classified adverts. Some of the finest ones are Craigslist, Kijiji (owned by eBay), BackPage, and Hoobly but you can see plenty more if you do a search on any search site. Sites vary in what the free membership includes and you can always upgrade to a nominal monthly charge if you feel that it's a necessity to get the features you would like.
In any case, post as many classified advertisements as the site allows. You should change the announcements up to use different keywords and market areas areas that your business deals with. If you can go in every day and just change one letter or make a tiny adjustment to your ad, it'll remain close to the top because the site will view it as something new. You'll likely have to go thru a tiny bit of trial-error with many alternative adverts before you start to see what it really working and what isn't.
Another excuse that the Internet and advertising are so compatible is actually because your ad will have a hyperlink to your internet site. So your audience just clicks on the link and they are immediately at whatever it is that you want to offer. Manifestly, offline advertising does not offer this easy transition. Even if you capture your audience's interest, they may simply forget your internet site when it comes time for them to log on.