A warm is important to warm up your muscles and get them ready for the workout. Warm-up for 5-7 minutes.
If you are sweating, you are burning calories, and that is the key to burning fat. Your body will burn sugar calories first, and fat calories last. Your cardio workout should last 30 minutes, it typically takes that length of time to get into fat-burning mode.
It is not good for your heart just to abruptly end your workout; you need a cooling down period of 5-7 minutes of gradually decreasing the intensity of your workout.
This is important to keep muscles and tendons limber. It is a good idea to stretch after ever workout, when your muscles are warm.
Here are some good workout videos, a few of these I have shared in previous posts, but repetition can be a good thing.