What dosage of iron and biotin supplements should i take if i'm planning to take them together everyday...??

At 18 years One probably menstruates and that could be heavy and cause Anemia making iron supplementation helpful. Biotin is in the B complex group and has multiple functions including Blood Sugar regulation but too much of it has significant side effects also. Best to see the PCP/GP to help navigate the supplements especially if One's Diet /Exercise and Life Choices are Healthy.