Hi,
I am not an expert but I thought I would share what little I know. The bigger flowers are male and the smaller ones are female. I know that few varieties of begonia like the non-stop variety can be grown from seeds but it is very cumbersome and needs experience and a lot of patience. You should rather wait for the flowering to be done and collect the tuber at the end of the season. Hope that was helpful.Here is a link that might help:

http://www.gardenguides.com/how-to/tipstechniques/flowers/begonias.asp

What does the tuber look like?