My dear Friend,
You are only 18, so do not worry, you can still increse your height. Please follow the instructions given below.
1. Do regular physical excecise, along with yoga asanasa, particularly Tadasana, suryanamaskar, sarvangasana, dhanurasana, you can learn these by watching videos on you tube, but initially be careful, do not try to do too much, do slowly and then increse the repeatitions and time.
2. Eat a diet rich in
calcium ( milk and milk products), along with this take any tablet of calcium every day.
3 Eat diet rich in protiens that is egg white, protien powders, mushrooms, brocolli.
4. Sleep for at least 8 hours every day as some hormons those are very important for height gain are secreted only in sleep, so if you sleep less you will get less quantity of these hormons.
I wish you a healthy life.