Media planners, also known as brand planners or brand strategists, work at advertising agencies and create clients' ad campaigns. The media planners interact somewhat with the creatives -- i.e. the copywriters creating the ads and the ad copy -- but predominantly with the clients making decisions about how a media campaign will unfold. A big part of the media planner’s job is to pick the right kinds of places (the correct TV shows and magazines) to place different ads so that the client's product (and brand) is advertised to the correct audience.
You need a specific training or a graduate/PG degree to become a media planner as many agencies do require a college degree. Ad agencies offer entry level positions in the area.
The main thing a media planner needs is a willingness, and eagerness, to learn about the advertising world. The job can be very social, because it entails working with clients, so an interest in socializing with colleagues and clients is important. Also key is an understanding of how marketing and advertising work. How can a client -- a company with a specific product -- best brand themselves? Media planners need to be able to devise strategies for branding and, to do so, they must know the entertainment world (what TV shows and magazines attract what audience) so they can place the ads appropriately. Media planners need to know which shows, websites, magazines and other purveyors of entertainment will draw the client's desired audience.
While a lot of media planners will learn about the intricacies of the advertising world by working at an agency, bringing an interest to this aspect of media is key to success in a job like this.