When you sit down to actually research and read up on the topic, you should try to progress through three stages in order to build, knowledge on the topic followed by sound arguments.
First, you want to answer the question “What is the topic?” This means finding background information. Learn about the topic, the key terms, and familiarize yourself with the context of the resolution on both sides of the debate. In this stage you are looking for more objective sources that will inform you rather than try to persuade you. These articles should inform you about the general topic as well as the two sides of the argument. Collect all types of articles that build your working knowledge of the topic.
The next question to attack is “What are my arguments?” In this stage you will look for evidence that makes claims about the topic but is not specific to one argument. For examples, let’s look at the April 2013 resolution: The continuation of current U.S. anti-drug policies in Latin America will do more harm than good. A claim that is general about the topic would be that U.S. anti-drug policies in Latin America drive up the price of drugs and the cost of drug-trafficking. A claim specific to that argument for the Pro would be that this further enriches cartels and allows for the purchase of arms. A claim specific to that argument for the Con is that higher prices deter the consumption of drugs, which decreases the drug market in the US. Here you are looking for opinions and differing perspectives.
The last question to consider is “What are my impacts?” Having acquired working background knowledge and ideas about arguments, this stage is about finding evidence that specifically supports the claims you want to make.