HELP with my afghan

[melanied]

I just learned how to knit. I can cast on, and do the general knit. I have made 2 scarves and would like to try to make a blanket, (afghan) however I just don't understand how you get the length and width. When I knit the scarves I stayed working on the needles and I know with the size of the blanket all the work won't be done in one small line like a scarf. Please help me!!!