Is online advertising worth the money?

We have a really tight budget right now, but we still need to advertise or else no one will know about us and if that is the case no one will come to our website and buy our products. Since we are mainly a web-based business, we are condisdering starting to advertise on the internet. However, we are wondering whether or not online advertising is really worth the money. Does this type of advertising really bring in more business? It seems like if our ads were well placed we might be able to reach a lot of people, but we don’t know how to go about this.