Gertiegumdrop has given great advice, so read through it a couple of times. I'll give my two cents regarding protein powders...
Firstly, eating before or after a workout:
In my opinion, fasted cardio isn't really logical or effective. You need a good meal about 60 - 90 minutes before a workout, for energy. Without this energy, you won't be able to work as hard and will be exercising at a sub-par level. You want all the energy you can get to give 100%. Something slow-digesting such as porridge is an excellent choice for this pre-workout meal.
Following exercise it is essential to consume a meal of carbohydrates and protein. Exercise actually breaks your body down, damages your muscles. It is the recovery from food and rest FOLLOWING exercise that will make you burn the calories, develop lean muscle and get fitter. Therfore, the nutrients of a post-workout meal are very important. If you continue to exercise and not eat afterwards then you will keep 'breaking your body down' and it may hinder your progress in gettting fitter.
As for protein powders...
I would recommend a high protein diet when looking to burn fat. Protein requires the body to burn more calories to digest it than fats or carbs, and as mentioned by Gertiegumdrop, is required for helping those muscles recover.
Gertie is also right on that it is better to get your protien requirements from whole foods whenever possible. Fish, white meat, low fat cottage cheese, and boiled eggs are always great choices. I only usually recommend you go out and buy expensive protien powder is for some reason you cannot consume whole foods, or your job makes it difficult to eat regular meals (For example, I'll take a meal replacement if I'm working an 8 hour shift).
For the means of fat loss, I would not however say protein powders will offer much benefit in relation to what they cost to buy. Stick with focusing on getting a good diet in order.