Make a list of all the issues that interest you and which relate to cardiovascular fitness. For example, issues like cardiovascular fitness and life expectancy or cardiovascular fitness and Olympic athletes are all valid sub-topics. Alternatively, you can make a list of issues which pertain to this topic, which you don't understand and require further research.
Research the issue pertaining to cardiovascular fitness that interests you the most. Write down your discoveries, as well as subjects within this issue of cardiovascular fitness that interest you most. For example, if you decided to research cardiovascular fitness for weight loss, you might be interested in biological reasons that such exercise helps an individual lose weight. Alternatively, you might be interested in the most effective forms of cardiovascular fitness for weight loss.
Reread your research and construct an argument based on the area that interests you the most. This argument will become your thesis statement. Your thesis statement should be specific and present a topic that is open to debate. For example, you might write that cardiovascular fitness is a suitable preventative method for heart disease.
Write an introductory paragraph where you discuss the issue that your paper is about, concluding with your thesis. You may chose to start this paper with a quote from a doctor, athlete or other expert who works directly in the field of cardiovascular fitness. Starting with a quote gives your paper instant credibility and engages the reader.
Start your second, third and following paragraphs with your strongest arguments and facts which support your thesis. All this information will come from the research you've already done. For example, if your thesis is about how cardiovascular fitness benefits mental as well as physical health, you could start a paragraph stating the cognitive health benefits of older adults who engage in cardiovascular fitness.
Conclude your paper by restating your thesis on cardiovascular fitness. Touch briefly upon the points you've made, summarizing your paper with a meaningful statistic which supports your thesis. Alternatively you could end your paper with a quote or relevant anecdote.