Might want to invest in a pair of pruners that are sharp on both ends because the regular kind actually crush the branches when you cut them. Anyway, just picture in your mind what you want the tree to look like and prune the excess branches off.
Better to water your tree only when it needs it so, check the soil with a toothpick (from the bottom) by immersing the toothpick into the soil. If there is soil on the toothpick, the plant does not need to be watered.
Also, a good way to water bonsai is to immerse the pot in water and let sit for about half an hour to an hour. Your tree really shouldn't need this more than once a week but, check using the method described above.
A good bonsai book will be helpful to. Bookstores are abound with them. Also, read the bonsai articles in this website. They are very informative.
I personally love ficus trees, enjoy your to the fullest!