You are 18yrs and growing fast. This is the reason for your stretch marks. Your skin collagen fibres cannot keep up with your growth spurt. Could you share a picture please ? If these marks are itchy and exuding any moist secretion, then consult a dermatologist. If not, you can use Contractubex cream by applying it firmly on the stretch marks. If these are very itchy you may apply triamcinolone acetonide ointment very very sparsely but I won't recommend it at your age though it's a mild steroid. Better take a newer anti histaminic like desloratidine during the itchy days. May apply lacto calamine lotion if you don't have allergies to it.

Hello can you please share the picture.