There are many different ways to train but get the following elements correct and the results will follow:

Intensity how hard you need to work
Frequency how often per week you train
Duration how long each session is

You can not do high intensity followed by high intensity without adequate rest unless you want your performances to tail off. During such workouts the body will be pushed beyond what is comfortable, muscle tissue is damaged as muscles contract and time is needed usually 24-48 hours for repairs. Resting is therefore an essential part of any programme because hard sessions = repairs required = increase in growth and fitness.

 Frequency is much easier, you need to work out at the very least every other day . If you are a serious trainer then 5-6 times a week, with 2 high intensity sessions, 1 aerobic threshold, 1 recovery and the 2 other sessions specific to your needs. If you want to get fitter then 4 times a week is the minimum.

 Duration is much harder because of the intensity. To improve aerobic or CV fitness then it needs to be at least 15 minutes. Endurance athletes will need to go for a great deal longer as typically there events are more than 1 1/2 hours - with this type of training the intensity will increase if they are looking to improve their pace. Resistance sessions should be 1 set of 8 - 12 reps for 8 different muscle groups to start with and for more experienced trainers use the different resistance work grous in Fixed Weight article.