Things like Mars bars will give you a short boost which will wear off, while foods like bananas and apples will give you a longer energy boost.
In the long term, it may be worth getting your iron levels checked, and then either taking iron tablets or changing your diet to include more red meat, and dark green veg (spinach, broccoli, baked beans etc are good sources of iron).
This happened to me and as soon as I upped my iron intake i began to feel better
Slow releasze carbohydrates ill give steady level of energy throghout the day - if youeat a sugary high sugar hing this will give you a sharp high in energy but you will dip suddenly and crash low later. Porridegeis one and baked beans are noe too - can't thinnk of anymore right nowe