Have a look here
http://calibre-ebook.com/download
Download it and play with it for a while - particularly the conversion settings.
I use a base font of 13 and set the margins to zero.
If you want to change the font type work through the sticky in the epub forum
https://www.mobileread.com/forums/showthread.php?t=36361
As a newbie I found this tricky to work out but I got there in the end.
This should set you in the right direction
