chris bowcutt

hi, im new in this forum, and i've been taking an ap music theory class for my senior year, and i have a problem with composing. I can write themes (eight measures or so) but i have a problem. I don't know how the rest of the musical structure is. I know the basics of form (binary ternary etc...) but its hard for me to turn eight measures into 100 measures or over. Could anyone please give me some tips or insight, or direct me to a website that could help me?