How can I bulk up? I eat normally what an 18 yr old eats but my body does not gain weight.

See Dr., then eat... If one is truly underweight (based on height, family background, and body type), a dr. Can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while also doing weight-training to increase muscle. Proteins: egg white, chicken breast, salmon, ... Better fats: olive oil, peanuts, almonds, tofu, ....