Volumise hair ?? helpp!?

i am 13 years old and I have thin hair at the top of my head down to about just below my ears. After that point, my hair kinda poofs out and it makes my hair look really weird and flat (my natural hair ugh = =) i use herbal essasnses, cant spell lol, straight hair shampoo and conditioner to try get rid of it but it dosnt, and ive tried teasing my hair a little but it dosnt stay. Plz give me any suggestions for volumised hair for going to school?? btw dont hav hairdryer :/