Before you start thinking about ad design, placement, and budgets you really need to address the basics:

1. What am I selling?
2. Who is my target audience?
3. Is my sales message clear?
4. Is my sales message motivating?
5. What media best isolates my targets? (eg. TV [cable/free-to-air], radio, magazines, online, event sponsorship etc)

Unless you've got those five things sorted your efforts will be in vain.

It's also worth remembering that the key point is 'who is my audience' and whilst there are still some very good deals to be had in online advertising compared to other media, for access to highly targeted groups who frequent very specialist sites or portals that command huge audiences then you should be prepared to pay (and sometimes a lot) for the privilege of putting your message in front of them.