The obvious ones are targeting by age and gender, for example 18-25 age group, or Males aged 25-35 and so on.
You would then target specific regions or areas where those groups are likely to either frequent or view, such as specific TV channels like Bravo or Football grounds and so on. It is no good targeting a known area of high under 50 residence to target retirement homes for example.
You need to decide who would be the likely group who would buy such a product, so targeting specific income groups and occupations, not much point targeting a new exclusive nightclub to the over 60's or a luxury sports car to low income groups for example.
You also need to consider the relevancy of the advertising to the target audience, does it tap into the current fashions and movements within your target audience. Use of language and wording is paramount, the use of digital formats also, not much point targeting a group who spend all their time on the internet by advertising in the Times for example.
Use of keywords to push your advertisement or brand link up the search engine logarithms and again wording designed to attract the demographic you are targeting.
That should get you started.