It was suggested to me to read the book "Before you cast a spell" and I've come to the part about the God/Goddess and I would like to know if anyone can tell me is when I attempt to do a spell who is the God/Goddess that I pray to? Is it the God/Goddess of the coven that I've joined or what?
It really depends on you! Witchcraft is just a craft, adding deities or leaving them out all depends on your own relationship/religion with the gods/goddesses. The whole duality god/goddess set up in today spells tend to be associated with Wiccans. So if you are Wiccan, than this is the point you would call you patron and matron gods. If you are apart of a strict coven, then yes you would use the coven's set deities.
If you are not Wiccan, or do not follow a particular religion, it really is all up to you. Some people may choose a god to invoke that has something to do directly with a spell (for example choosing Aphrodite for a love spell), while other traditions only invoke gods that they have a relationship with (even if said god don't rule over whatever particular spell you are casting.)
Anyway, to sum it up- it all depends on you. If you don't feel the need to call upon deities, you can call upon a spiritual guide or even your ancestors. If you do feel the need, sit down with yourself and think about what gods you want to ask for help in the spell and who you feel most connected with. I hope that helps a little. Good luck with your spell friend!