Whatever format you use matters little. The outcome really rests on whether you actually have well-thought-out grounds for taking your position that you can clearly explain and your ability to counter the opposition's statements. Before putting together your evidential basis for your reasoning, do you know what you need to accomplish to win?
In the real world, outside of academic debates, there probably will not be judges, just an audience with varying levels of knowledge of the subject. They may be voters you want to convert to support your candidate or they may be workers in a union discussing the pluses and minuses of a proposed contract. In many cases, you will not know the actual impact you have had at the end of the debate. Even if there is a vote at the beginning and at the end, partisans are unlikely to publicly admit to having changed theirs minds, although you might dampen their enthusiasm by raising disturbing questions about their positions. All you can do is give the process your best shot. That means keeping a few things in mind, especially within the limitations of a traditional debate structure.
If you have the affirmative and are advocating change, you must show that there is a serious problem that needs to be addressed and that your proposal is the best course to that end. You also need to anticipate how the negative side will criticize your positions and develop effective responses, which take into account the knowledge and attitudes of the audience (it does no good to “win” on intellectual points but persuade no one, which happens all too often).
If you take the negative position, one way to undermine the affirmative is to question whether the problem is as great as it states. Or, conceding that, you could argue that their proposal for change would either not solve this to the extent they claim or would have such negative side effects that it would not be a good trade-off. It is not necessary to offer an alternative plan, but not offering one would likely convince the undecided that you are simply a naysayer without a solution.