Hey ShiLing Star
I noticed that you ended up being the one doing the assisting in this post rather than being assisted, so I'm here to help with your original question if you still need it.
When you add Pages to a given menu you will notice that you can drag and drop them up and down to change the order. If you drag a page to the right, it will shift into an indented slot, kind of like pressing tab on a keyboard. You have now created a secondary option on your menu which can be viewed by scrolling over the menu option listed above.
So just to clarify, if my menu included fitness, sports, outdoor, and indoor sections, and I wanted to create a scroll down menu for the sports section, my page bars in the menu section of the wordpress theme would look like this:
Hope this helps