Target Group Definition is the most important to do, BEFORE you start advertising.

Make sure you know who you want to see your advertisements. Otherwise you are wasting money!
One of the great things with online advertising is, that you can target people in a certain area - country, zip-code, radius around your store...

Search Engine Marketing with Google AdWords lets you target the right people and get the most BANG for your buck.

Facebook marketing is a good alternative, if you want to target people through demographic facts instead of targeting them on their search for certain products.

Always define your target group accurate and precise.
Check out this video to learn what exactly this means.