First off, welcome to the forum! =)
Now, in order to gain weight the basic law is: energy intake > energy expenditure. So, you need to be taking in more calories (eating more food) than your body is burning off.
Being 5'8 and 57kg, that's pretty thin and sounds like you have an extremely fast metabolism. So in my opinion you need to be taking in about 2200-2500cals every day. Now, in terms of putting on weight - you need to be eating 5-6 meals per day aiming at about 400cals per meal. Each meal should contain carbohydrates, proteins and fat.
Foods with quality carbs:
- Brown rice
- Multigrain/brown bread
Foods with quality protein:
- Red meats: lamb, beef
- Skim Milk
Foods with quality (unsaturated) fats:
- Eggs (yolk)
- Unsalted peanuts
You could also look into a protein supplement for after training and before bed to assist you however it isn't a necessity.
Hope this helps, if you need anything just PM me =)
P.S. with exercise - keep doing what your doing, cardio keeps you fit/healthy and lifting weights will help you gain muscle mass. The only thing you need to ensure is that you eat more than your body burns off.
Thanks to limba for this AWESOME sig