How to start working out

Diet

First of all is the diet. Why?
Because due to the diet you will transform from skinny to muscular.
Now what you need to do... Try to reduce the quantity of the junk food you eat. Eat healthier! It doesn't mean you can't eat that kind of food, but just reduce it. For example i have a cheat meal per week, when i can eat everything i want. At first it will be difficult but you will get used to it.
Next, when you get used to it you have to start eating smartly. Before you have your workout(preworkout), you need to eat carbohydrates(carbs.) like potatoes, rice, pastas etc. because you will need the energy, that is given by the carbs for training.
After your workout(post workout), you just have to eat a lot of products that have a big quantity of protein and calories; recomanded: egg whites, beef, pork, etc.
Last but not least, you don't have to look at the food like if you were a little kid, as a reward that you got due to the action that you were good, you have to think about it as a fuel that will keep you going trough your training and your day, that will make you give everything at the gym/street workout.