Start by asking critical questions as to why advertising is portraying something in a particular way.
it could be a particular identity; i.e. gender, age, race, class, sexuality, able/disabled bodies, Canadian identity
it could be a particular issue or topic; i.e. education, family, religion, science, aging, celebrity, politics, motherhood, love,
it could be a particular type of product/activity i.e. smoking, cars, alcohol, gambling, condoms, organic food, children’s food
What is happening socially, culturally, politically or economically that would warrant such a portrayal? What does this portrayal reveal about contemporary society. You may want to ask how such a portrayal reinforces or contests particular ideologies?
You could look at the ways a particular advertising campaign, or issue, or advertising medium, promotes the ideology of consumer culture.
For ideas on topics look up the Journal of Consumer Culture. There are lots of great scholarly articles in there.
Note: DO NOT write an essay about the effects of advertising. You do not have the resources to be able to adequately assess how a particular advertising campaign has an immediate identifiable effect on an individual or population. Also effects based research is really, really problematic.