If you are racing as a junior (which you can do until age 18) then you might find combined fields with both men and women, but I believe that they are always scored separately. In all likelihood you will end up racing in the women's field most of the time though - usually there are a limited number of races that have Juniors fields.

When/if you go to college you can race collegiate and the races will all be separated.

Now - you do have the option of racing with the men and are allowed to race one category down from your normal one if you choose. Sometimes I see women do this when they feel the need for challenge or if there is not a women's field. In this case you would not be scored separately. Often you'll see promoters give women and sometimes juniors a reduced entry fee if they have to race with the men.