Eat primarily complex carbohydrate from things like pasta, rice, bread (multigrain is best overall) and potato. Carbohydrates are what give you most of the energy your body uses and stores, that can be accessed pretty quickly. In all cases the more natural varieties are better, so whiter ones that have usually been modified have had some of the nutrients removed should be eaten less often. This doesn't mean never.

After that, things like green leafy vegetables are great because they're high in iron which helps with energy production. Other types of veggies and fruit are vital too as they give you vitamins that also help in energy production.

If you feel lethargic, don't be tempted to have a lot of something sugary, have a sugar based food but in a small quantity and wait to see if it works before having more. Jelly beans are great to have around for this.