Height of an individual is determined by Genetics, Environmental & Nutritional factors and Hormonal
factors (most important the Growth Hormone
secreted by pituitary gland
An individual stops growing in height after the fusion of the Epiphyses (growth plates) of the long bones, which occurs by the age of 17-21 yrs in males.
So...as you are 18 yrs old now, you still have a good chance of increasing your height.
An X ray of your wrist joint can be taken to look for the fusion of epiphyses.
Make sure that you are taking a healthy & balanced diet, exercise regularly & avoid stress.
Protein rich diet like soya, groundnut, pulses, etc should be incorporated in your diet plan
One more thing...avoid consuming any over the counter medication
which claim to boost your growth hormone & increase height...it may be harmful.
Wishing you Good Health.