When I draw a river, it usually only shows the main flow from its source to the end. But I also need to draw tributaries. How can I do that? Like, where do I start drawing them from?

If you are building your model manually, you can create as many tributaries as you like by dragging the river symbol onto your WEAP area, clicking to draw the tributary and double-clicking at the confluence with the main river to finish. Starting in exactly the right place is not that important--you just want to make sure that the inflows and outflows are in the correct order throughout your model.
If you are using Catchment Delineation Mode, you can adjust the River Detail slider in the Catchment Delineation Mode dashboard. This will show more tributaries.

Anne